Text content (OCR)
MAY 1980 Volume 5, Number 5 $2.50 In USA/$2.95 in Canada
EVIE
the small systems journal
A MCGRAW-HILL PUBLICATION
Think BIG
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The intelligent controller, using DMA data transfer, makes
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CDS-1 “‘Winchester’’ disk drive with controller . 63,995.00
Cabinet—matching our 6809 computer desk... $150.00
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219 W. RHAPSODY
| 4 SAN ANTONIO, TEXAS 78216 (512) 344-0241
Gircte 325 on Inquiry card.
‘¢ Cromemco
11 megabytes of hard disk and 64 kilobytes of fast RAM in a
Z80A computer for under $10K. Two floppy drives, too.
Naturally, it’s from Cromemco.
It’s a reality. In Cromemco’s new
Model Z-2H you get all of the above
and even more. With Cromemco you
get it all.
In this new Model Z-2H you get
not only a large-storage Winchester
hard disk drive but also two floppy
disk drives, In the hard disk drive you
get unprecedented storage capacity
at this price—11 megabytes unfor-
matted.
You get speed—both in the 4 MHz
Z80A microprocessor and in the fast
64K RAM which has a chip access
time of only 150 nanoseconds. You
get speed in the computer minimum
instruction execution time of 1 micro-
second. You get speed in the hard
disk transfer rate of 5.6 megabits/sec.
EXPANDABILITY
You get expandability, too. The
high-speed RAM can be expanded to
512 kilobytes if you wish.
And the computer has a full 12-slot
card cage you can use for additional
RAM and interface cards,
BROADEST SOFTWARE SUPPORT
With the Z-2H you also get the
broadest software support in the
Gireta 1 on inquiry card.
microcomputer field. Software Cro-
memco is known for. Software like
this:
Extended BASIC
FORTRAN IV
RATFOR (RATional FORtran)
COBOL
280 Macro Assembler
Word Processing System
Data Base Management
with more coming all the time.
SMALL, RUGGED, RELIABLE
With all its features the new Z-2H,
including its hard disk drive, is still
housed In just one compact cabinet.
Included in that cabinet, too, is
Cromemco ruggedness and reliability.
Cromemco is time-proved. Our
equipment is a survey winner for
reliability. Of course, there’s Cro-
memco’s all-metal cabinet. Rugged,
solid. And, there’s the heavy-duty
power supply G0A @ 8V, 15A @
+18 V, and 15A @ —18V) for cir-
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plug into those free card slots.
SEE IT NOW
Last summer we told you this new
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So see it at your dealer’s now. Have
him put you in touch with a user—
there are lots of them because
Cromemco has been delivering for
months, See for yourself how pleased
our users are.
PRESENT CROMEMCO USERS
We've kept you in mind, too. Ask
about the new Model HDD Disk
Drive which can combine with your
present Cromemco computer to give
you up to 22 megabytes of disk
storage.
fneorporated
280 BERNARDO AVE., MOUNTAIN VIEW, CA 94040 © (415) 964-7400
Tomorrow's computers today
‘2 BYTE May 1980
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‘Expandability
‘Standard Bus
for System
BUTE may 1980
Volume 5, Number 5
la The Queue
Foreground
20 A DC-TO-DC CONVERTER by Michael Picco
Here's a simple converter that uses a standard integrated circuit for producing a 25 mA bipolar
source from a single-ended power supply.
22 I/O EXPANSION FOR THE RADIO SHACK TRS-80, Part 1: Principles of
Parallel Ports by Steve Ciarcia
This month Steve explains the operation of parallel input/output as a prelude to next month’s
design for an economical RS-232C interface.
44 KIMDOS, Using Your KIM-1 with a Percom Floppy-Disk Drive by Joel Swank
Using the LFD-400 disk-controlier board, the KIM-1 can access up to 87.5 K data bytes on several
5-inch hard-sectored floppy-disk drives.
72. INTERFACE A FLOPPY-DISK DRIVE TO AN 8080A-BASED COMPUTER
by John Hoeppner
Building a disk-controller board for a Shugart SA400 disk drive can be done easily and with com-
monly available parts.
196 GIVE YOUR COMPUTER AN EAR FOR NAMES by Tom Munnecke
With the Soundex code, you can locate people's names in your data base by similar, but not ex-
act, spellings.
214 THE COSMAC DOODLER by Jeff Duntemann
An electronic sketchpad? Even a small system like the COSMAC ELF can draw designs using a
video display,
250 ERROR CHECKING AND CORRECTING FOR YOUR COMPUTER
by Gregory ] Walker
Storage devices can introduce data errors, The system presented here can increase reliability and
speed of these peripherals,
Background
12 THE CASSETTE LIVES ON, An Alternative to Floppy-Disk Mass Storage
by Emory Cook
Floppy disks may be the glamorous way to store programs and data, but the cassette is far from
obsolete,
104 A GRAPHICS TEXT EDITOR FOR MUSIC, Part 2: Algorithms
by Randolph Nelson
The conclusion of this article sets forth the routines to create and use the various arrays described
in part 1.
120 USING THE COMPUTER AS A MUSICIAN'S AMANUENSIS, Part 2: Going
from Keyboard to Printed Score by Jef Raskin
Part 2 continues the examination of the subtle problems encountered when translating information
from performance to written score,
130 COMPARING FLOPPY-DISK DRIVES BY SOFTWARE SIMULATION
by Dennis Nendza
Now you can get some idea of the relative performance of different units by simulating their
mechanical functions in a BASIC program.
202 THE CLUB COMPUTER NETWORK by Joe Kasser
If your club is considering to form a program- and data-exchange network, the telephone and.
amateur radio links described here will be a valuable source of ideas.
Nucleus
6 Editorial: Computer-Controlled ut for the KIM-1
Viewing of the 1980 Eclipse 226 Book Reviews
8 Letters 230 Clubs and Newsletters
144 BYTELINES (formerly BYTE News) 234 BYTE's Bits
152 Technical Forum: Simplifying the 236 BYTE's Bugs
Curve-Plotting Calculation by 238 Event Queue
Geometric Means; Alpha Locking in 280 NCC Information
Software; Maintaining a Single Exit 286 What's New?
Point 335 Unclassified Ads, BOMB Results
190 Programming Quickies: Decisions, 336 Reader Service, BOMB
Decisions; Formatted Program Out-
May 1980 © BYTE Publications Inc 3.
Publishers
Virginia Londoner, Gordon R Williamson
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4 May 1980 © BYTE Publications Ine
ON THE COVER
May's cover, Robert Tinney has formed an abstraction
of the most important medium of mass storage in today's era
of small computers, the floppy disk. Heightening its shimmer-
ing mystery, we find a disk wavering in the heat above some
desert landscape. To enlighten you, this issue features several
articles that present valuable information about floppy-disk
technology. This technology is no mirage — it will even work
well in a similar, hot environment of East Africa, as the
editorial on page 6 describes.
fin 51 Maen is nln Conroy Bala FA atin, Peta datl ebiva yi bisa sri a
E. Boddort, Gene Daniel A. McMillan; Senior Vice President-Edit
‘Schulz; Vice
Robert B, Doll, Circulation;
Planning and Development; sohn W. Pate
dE. Schirmer, Intemational.
jent, Chief Executive Officer and Chaitman of the Board;
jaiph J Webb, Treasurer.
J
pobert F Landes, Senior Vice President and Secre tary;
BYTE Is published monthly by BYTE Publications Inc, 70 Main St, Peterborough NH 03488, a wholly-owned sub-
sidlary of McGraw-Hill, Inc. Address all mail except subscriptions to above address: phone (603) 0240281. Address
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United States funds drawn on & US bank. Printed In United States of America.
Address all editorial correspondence to the editor at the above address. Unacceptable manuscripts will be
retumed if accompanied by sufficient first class postage. Not responsible for lost manuscripts or photos. Opinions
expressed by the authors are not necessarily those of BYTE. Entire contents copyright © 1880 by BYTE Publica-
tlons Inc. All rights reserved.
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Altos Computer
2360 Bering Drive
San Jose, CA 95131
Apple Computer
10260 Bandley Drive
Cupertino, CA 95014
‘Commodore Business Machines, Inc.
3330 Scott Boulevard
Santa Clara, CA 95051
Digital Microsystems Inc.
18 Piedmont Avenue
Oakland, CA 94611
Industrial Micro Systems
628 N, Eckhoff
Orange, CA 92668
Micromation inc.
1620 Montgomery Street
San Francisco, CA 94114
Morrow Designs inc.
Thinker Toys
5221 Centrat Avenue
Richmond, CA 94804
North Star Computers Inc.
1440 Fourth Street
Berkeley, CA 94710
Polymorphic Systems
460 Ward Drive
Sonto Barbara, CA 93111
Yano Corporation
4301 Poche Court West
New Orleans, LA 70129
Technico inc.
9051 Red Branch Road
Columbia, MD 21045
Texas Electronic Instruments
5075 S. Loop East
Houston, TX 77033
Vista Computer Company
4401 E. Borchard
Santa Ana, CA 92705
“..Shugart
6 — May 1980 © BYTE Publications Ine
Computer-Controlled Viewing
of the 1980 Eclipse
by Carl Helmers
As noted in the March 1980 editorial, I traveled to Kenya in East Africa to
observe the 1980 total solar eclipse with an Apple II Pascal system controlling
the photographing of the event. This month’s editorial is a commentary about
the experience, This commentary was written upon my return to New Hamp-
shire a week after the eclipse,
The final preprations for the Kenya eclipse of 1980 were made in an inten-
sive session of 24-hour workdays, February 2, 3, 4, and 5. One physiological
consequence of no sleep for 3 or 4 days is that when traveling through 8
time zones there is no possibility for jet lag! One’s body is so tired that all
memory of the previous time zone is erased completely. Norm Whyte and
Laurel Allen, who coordinated many of the details of the trip to Kenya, arri-
ved in Boston from California on the second of February and spent the
weekend at my home. During this final weekend's activity, we each had
several chores to finish. One detail, for example, was making sure that both
computers would operate simultaneously off Norm's portable Honda AC
generator, Another was adding a hardwood extension to Norm’s telescope
mount so that my camera could be attached along with his.
In connection with the program design of my experiment, a number of
crucial points had to be verified. With the time allocation procedures com-
pleted as described in the March 1980 editorial, writing the real-time pro-
cedures to execute the time line proved trivial. These were the procedures left
in dummy form in the listing 1 published with the March 1980 editorial, In
listing 1 accompanying this editorial, readers will find the final form of the
program I used. In approaching this final form I implemented the execution
routines using a module named “milli” to carry out time delays of an integer
number of milliseconds. The program itself was verified by driving the camera
interface using a first approximation to “milli” in the form of Pascal dummy
loops used to count time.
Originally I hoped that (by fortuitous circumstance) I could use some com-
bination of Pascal statements in a loop to provide time delays in units of
milliseconds. But, after perhaps an hour of fooling with various combinations,
I came to the conclusion that this would not be possible, I was either 6% too
slow or 6% too fast depending on whether or not I put a unary negation in a
timing loops dummy assignment statement.
Since program development time was limited by a departure schedule, it
soon became apparent that the lesser of two evils (imprecision or assembly
language) was to write an assembly-language routine called “milli” that links
to Pascal with a single integer parameter specifying a loop delay time in
milliseconds. I finished this necessary step sometime in the wee hours of
February 4. I checked the accuracy with various simple test programs written
in Pascal. Of course, my timing assumption was that zero time would be spent
outside of “milli” executing the Pascal code of the actual program. This
assumption was verified with test runs of the whole eclipse photography se-
quence, which showed about 1% error. By adjusting the constants in the delay
routine slightly, this error was compensated at the gross level of the entire
eclipse sequence's 241-second execution time.
Text continued on page 52
“After working all day with the computer at
work, it’s a kick to get down to Basic at home. And
one thing that makes it more fun is my Shugart
minifloppy™: We use Shugart drives at work, so
when | bought my own system | made sure it had a
minifloppy drive.
“Why? Shugart invented the minifloppy. The
guys who designed our system at work tell me that
Shugart Is the leader in floppy design and has
more drives in use than any other manufacturer. If
Shugart drives are reliable enough for hard-working
business computers, they've got to be a good
value for my home system.
“When I’m working on my programs late at
night, | can’t wait for cassette storage. My
minifloppy gives me fast random access and data
transfer. The little minidiskettes™ store plenty of
data and file easily too.
“| made the right decision when | bought a
system with the minifloppy. When you lay out your
own hard-earned cash, you want reliability and
performance. Do what | did. Get a system with the
minifloppy.”
Af it isn’t Shugart
it isn’t minifloppy.
4 Shugart
Oakmead Parkway, Sunnyvale, Califomia 94086
See opposite page for list of manufacturers featuring Shugart’s minifloppy in their systems.
TM minkloppy Is a registered trademark of Shugart Associates
Information on
Potter Printer Needed
Can a reader of BYTE help me? I
recently purchased a printer from
salvage, and I hoped to obtain
documentation and a schematic diagram
from the manufacturer,
The printer is a Potter Model
LP-3000, manufactured by the Potter In-
strument Company, formerly of Plain-
ville, New York. I called the firm, and I
was told:
1) The company is in the process of
moving to New Hampshire.
2) This particular model of printer is ob-
solete.
3) They have no documentation or
schematic for this model.
From my examination of the circuits
and machinery, I believe the Potter
LP-3000 is a daisy-wheel type with a
serial data input. However, whether it
uses ASCII or not, I can’t tell.
Can someone tel! me how I can inter-
Letiers
face this printer to my Radio Shack
TRS-80 Model I Level Il computer with
expansion interface?
Nick Tountas
838 Juniper Rd
Glenview IL 60025
Questioning ‘‘Affordable’’!
When you are on Social Security, an
affordable computer system that costs
$6000 is like “# @ * “I When your
monthly income is $360, to have an
editor smugly talk of plunking down
$6000 cash as if it were a minor outlay
tends to be very irritating.
On top of that, the system Mr
Helmers described is just the sort (with
minor modifications) I have wanted for
ages. Another thing that hurts is the
industry-wide disinclination to even con-
sider time payments or credit. I know
that I'll have to wait, and probably wait
over 5 years, but maybe not. If 1 were
just disgusted with your editorial, 1
wouldn’t have bothered to write. What 1
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8 — May 1980 © BYTE Publications Inc
Circle 2 on inquiry card.
would like to know is if any BYTE
reader knows of a way I can obtain such
a system as Mr Helmers described —
perhaps secondhand — without paying
thousands of dollars cash? By squeezing,
I can afford $100 a month now, and by
July I should be able to afford $150 a
month, perhaps more.
In a way, I have to thank Mr Helmers
for that editorial. It made me mad
enough to write, and perhaps there is a
solution to my problem.
Fred J Remus Jr
POB 2453
San Diego CA 92112
Carl Helmers Replies
Give the industry time. Five years
ago, the same system might have been
well in excess of $30,000, with inferior
programming languages and comparable
on-line storage capacity. Tremendous
strides have been made in the past 5
years, and we can expect a certain
leveling-off of prices in the future as
mass production at 100,000 unit levels
per year starts becoming reality. And
then, of course, one looks at it from the
point of view of increasing demand for
these products. If we do not write about
the conception of a good machine, we
have no interest on the part of
users..,.CH
Gomoku
I was interested in the “Programming
Quickie” by John Allwork (“BASIC
Game: GOBANG,” November 1979
BYTE, page 56) for Gobang is also call-
ed “Gomoku.” There has been a com-
petition running for Gomoku programs
since 1975; | am the current champion.
People interested in the contest should
contact:
Shem Wang
Dept of Computer and Information
Science
University of Guelph
Guelph, Ontario, CANADA
So far my different programs have run
‘on large mainframe computers, but I
hope to have one working on my North
Star microcomputer for the next round
of competition.
Mike Compton
196 Metcalfe St, Apt 810
Ottawa, Ontario
K2P 1P8 CANADA
At Intersystems,
“dump” is an instruction.
Not a way of life.
(Or when you're ready a IEEE S-i00
computer be ready for you?
We're about to be gadflies again.
While everyone's been busy
trying to convince you that large
buses housed in strong metal
boxes will guarantee versatility
and ward off obsolescence, we've
been busy with something better.
Solving the real problem with the
first line of computer products
built from the ground up to con-
form to the new IEEE S-100 Bus
Standard. Offering you extra ver-
satility in 8-bit applications today.
Anda full 16 bits tomorrow.
We call our new line Series
lI And even if you don’t need the
full 24-bit address for up to 16
megabytes (!) of memory right
now, they’re something to think
about. Because of all the perform-
ance, flexibility and economy
they offer. Whether you're looking
at a new mainframe, expanding
your present one or upgrading
your system with an eye to the
future. (Series Il boards are com-
patible with most existing S-100
systems and all IEEE S-100 Stan-
dard cards as other manufacturers
get around to building them.)
Consider some of the fea-
tures: Reliable operation to 4MHz
and beyond. Full compatibility
with 8- and 16-bit CPUs, pe-
ripherals and other devices. Fight
levels of prioritized interrupts. Up
to 16 individually-addressable
DMA devices, with IEEE Standard
overlapped operation. User-selec-
table functions addressed by DIP-
switch or jumpers, eliminating sol-
dering. And that’s just for openers.
The best part is that all this
heady stuff is available now! In
our advanced processor—a full
IEEE Bus Master featuring Memory
Map™ addressing to a full mega-
byte. Our fast, flexible 16K Static
RAM and 64K Dynamic RAM
boards, An incredibly versatile and
will your
2)
economical 2-serial, 4-parallel
Multiple 1/O board. 8-bit A/D-D/A
converter. Our Double-Density
High-Speed Disk Controller. And
what is undoubtedly the most flex-
ible front panel in the business.
Everything you need for a com-
plete IEEE S-100 system. Available
separately, or all together in our
new DPS-1 Mainframe!
Whatever your needs, why
dump your money into obsolete
products labelled “IEEE timing
compatible” or other words peo-
ple use to make up for a lack of
product. See the future now, at
your Intersystems dealer or call/
write for our new catalog. We'll
tell you all about Series I! and the
new IEEE S-100 Bus we helped
pioneer. Because it doesn’t make
sense to buy yesterday's products
when tomorrow’s are already here.
(hotleenrQyyssthenonss”
Ithaca Intersystems Inc.,
1650 Hanshaw Road/P.O, Box 91,
Ithaca, NY 14850
607-257-0190/TWX: 510 255 4346
Searching for
FORTRAN Compiler
lam an avid reader of BYTE and I
believe that one of my fellow readers
may be able to help me with a problem.
My school is thinking about expand-
ing the courses that are offered in the
area of computer science. It is hoped
that an extensive course in FORTRAN
programming may be offered.
Our computer is a CIP/2200 manufac-
tured by the Cincinnati Milacron Cor-
poration. It has a small disk-operating
system and a card reader. The word size
is 32 bits, and, at this point in time, the
memory size 1s 32 K bytes, There are
plans, however, to expand the memory
to 64 K bytes by the time the FOR-
TRAN course is offered.
My problem i. that the Cincinnati
Milacron Corporation does not make a
FORTRAN compiler for our machine. I
would like to know if any reader of
BYTE could suggest any companies that
might sell a compiler that is compatible
with our machine.
Daniell B McCormick
Box 675
Presbyterian College
Clinton SC 29325
Seeking Computers for the Blind
Does any reader of BYTE know of a
source for a computer system that uses
audible output instead of characters
displayed on a terminal for its
customary interaction with the user,
such as that produced by the Votrax
speech interface? Such a computer
system would be used by blind people.
It would be desirable if a BASIC inter-
preter that used audible output were in-
cluded,
If anyone has or knows of such a
system, please contact me.
Walter F Keleher
56 Robin St
Rochester NY 14613
Altair BASIC Patch Needed
I wonder if any BYTE readers could
assist me in locating the patch to Altair
8 K 4.0 Version and Altair Extended 4.0
Version BASICs which will allow these
BASICs to run on a Z80.
I recently purchased a TDL ZPU
which uses the 280. The manual notes
It took the reaer of SWITCHED OW BACH 12 painstak-
ing years to realize every synthesized note of Bach’s
most popular works, the completa Brandenburg
Concertos. Hear a truly electrifying performance from
the best-selling artist in electronic music.
Wendy Carlos’ SWITCHED-ON BRANDENBURGS.
Aspecially-priced double album.
On CBS Masterworks Records and Tapes. \
“CBS; "Masterworks," @are ‘trademarks of CBS Inc. © 1980 CBS Inc.
10 May 1980 © BYTE Publications Inc
SprcialyPriced ‘Zager st7|
no prema
AUISER Rs
M2X 35895
this incompatibility stating that Altair
BASIC “has as part of its routines
several occasions where the parity flag is
checked as part of the function. In the
Z80 the parity flag indicates
OVERFLOW during math routines, not
parity.” The manual states that it con-
tains a patch in Appendix C, but no Ap-
pendix C is included.
If any reader of BYTE knows where
this patch may be obtained, please let
me know.
Hugh Morgan
7725 Berkshire Blvd
Powell TN 37849
Pascal Examples Needed
Just a short note to tell you how very
much I appreciated Carl Helmers’
“Pascal Checkbook Balancing Program”
which appeared in the January 1980
BYTE.
As a beginner, I don’t think he
"profaned Pascal by writing a simple
little...” etc. The program was most in-
formative, and I studied it in detail. [
have adapted it to the formulation of a
metrics conversion program. It was cer-
tainly clearer than most of the program
examples in the few, but confusing, texts
on Pascal.
T realize that in general BYTE
magazine caters to the experienced pro-
grammer, but what we need are more
examples like yours—the we being those
of us relatively new to the art.
So thank you once again—and please
some more tutorials and programs|
Max Nareff
5235 Diamond Heights Blvd
San Francisco CA 94131
A Satisfied Reader Comments
I couldn't believe it! Ted Carter's
article “Implementing ic Data
Structures with BASIC Files” (February
1980 BYTE, page 92) was exactly what I
needed for a program I am writing to
computerize billing on a newspaper
route.
] had tentatively planned my file
routines, but I scrapped my ideas after
reading the article.
James E Nichol
1416 Oak Knoll Dr
Cincinnati OH 45224 @
Circle 3 on Inquiry card. —>
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VB3 is memory mapped
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It generates both U.S. and
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The Cassette Lives On
An Alternative to Floppy-Disk Mass Storage
In a world where floppy and hard
disks are becoming more affordable
for the average small-business user
and hobbyist, sequential mass storage
in the form of cassette tape is gaining
disfavor, Still, many disk users get in-
to trouble when something happens
to a floppy disk and they have not
made backup copies. Although any
backup system requires the time and
inconvenience of regularly carrying
out the file-copying procedure, one
problem with using floppy disks for
file backup is the cumulative cost of
the number of disks needed to main-
tain backup copies of all records.
The Cassette Solution
What is needed is a low-cost filing
medium. Cassette storage is the
answer, provided we take the
Necessary precautions to make it
reliable. Old files, such as files of
records for last quarter, last year, and
the years before, belong on cassette.
The disk-to-cassette transfer for back-
up purposes becomes sensible in
terms of both expense and security.
With adequate tape recorders and
high-quality cassette tapes (which use
both quality tape material and quali-
ty mechanical housings) cassette
storage can and does become highly
dependable.
Let’s go a step further. Anyone
who is using a microcomputer and
needs its daily functioning will be
acquiring a spare microprocessor.
With a three-head, audio-cassette
12 May 1980 © BYTE Publications Inc
Emory Cook
Cook Laboratories Inc
375 Ely Ave
Norwalk CT 06854
machine,which has a separate
playback head following the record
head (a common piece of high-
fidelity equipment), the spare micro-
processor can readily be set up with a
machine-language program. This
program verifies a backup tape by
reading the information immediately
as the tape is written. [The same
result can be accomplished (a bit
slower, however) for those of us who
cannot afford a spare microprocessor
board or an expensive cassette re-
corder. This can be attained by using
a verification program running on the
same microprocessor to reread the
newly created tape and compare its
information with the contents of
computer memory....GW]
Floppy disks may be a
glamorous way to store
programs and data, but
the cassette is far from
dead.
When records are backed up at the
end of some reasonable period (ie:
day, week, month, etc), the extra
time needed to dump the records to
cassette at a low transfer rate is not an
overwhelming disadvantage. A se-
cond backup tape simultaneously
made with a second recorder is
always a good idea. In other cases,
one cassette copy can simply serve as
a backup for printed records, thus
saving time, printer wear, ribbons,
and paper.
For even the most inexpensive
cassette deck, a small amount of
money and attention can result in the
following:
@ excellent performance and relia-
bility (no more trial-and-error
adjustment of the volume control)
®@ a very low error rate (statistically
as good as that of a 5-inch floppy
disk)
@ the lowest possible cost per bit
stored
Problems with Cassette Storage
The main problems with currently
used cassette-storage methods are
dirt, variation in tape speed, prob-
Jems with azimuth alignment, and
inferior tape quality.
Dirt collects on the tape recorder
head from several sources, from the
room, from dust left on poorly
manufactured tapes, and sometimes
from sweaty fingers attempting to
wipe the head clean. The tape head
and the pressure roller can be cleaned
using pure alcohol and a cotton-
tipped swab.
Periodic cleaning is imperative
when using poorly manufactured
tapes. Cassette tapes are manufac-
tured by slitting a 30.5 cm(12-inch)
wide sheet of magnetic material called
a web. Slitting is accomplished with
knives, which often get dull from cut-
Circle 4 on inquiry card,
ling the inherently abrasive magnetic
coating. If the knives are not
periodically sharpened (which is the
case in making some inexpensive
cassettes), the dull knives cause a fine
powder of magnetic coating to collect
on the edges of the tape. As a result,
abrasive magnetic powders come in
contact with the tape head when the
cassette is later played. The poorer
the quality of the tape, the greater the
chance that this is occurring.
Variation in tape speed can be
caused by belts slipping within the
cassette recorder, but it is more often
caused by flaws in the pressure roller,
which with the capstan is meant to
push the tape through the machine at
a constant speed. Leaving the tape
recorder set in play mode with the
motor disengaged (as is done in
several current personal computer
systems that let the computer control
the tape motor) may eventually cause
indentations on the pressure roller,
with some inevitable variation in tape
speed. This variation impairs the
reliability and the data-transfer rate
of the cassette interface, so it is im-
portant to keep the pressure roller
clean at all times and disengaged
when not in use.
Azimuth of the tape head refers to
the angle between an imaginary line
drawn in the direction of tape move-
ment and the vertical, magnetic gap
on the record/playback head of the
cassette recorder. This angle should
be 90°-that is, the tape should run
Number Type +9V GND
C1 8 4
straight across the tape head, per-
pendicular to the magnetic gap. If the
tape head is somehow knocked out of
alignment (which happens frequent-
ly, although nobody knows how), it
must be restored if the tape recorder
is to faithfully play back a recorded
tape.
There is an adjustment mechanism,
usually a small Phillips screw, on the
left-hand side of most tape heads.
However, some tape recorders do not
allow you to reach the mechanism
when the recorder is in the play
mode. Because of this, it is important
to do one of two things: either buy a
cassette recorder that has an azimuth
access hole, or have a good craftsman
carefully drill a hole over the screw so
that it can be reached with a tiny
screwdriver when the recorder is in
the play mode.
Recording with a Peak-
Signal-Strength Meter
It is the peak output, not the
average or the root-mean-square val-
ue of the cassette signal, that most
tape interfaces are sensitive to. In or-
der to repeatedly load cassette tapes
on the first try, you must be able to
send a signal of known strength to the
cassette interface. However, most
computer systems give us no feed-~
back on cassette signal strength—in
other words, we are operating “blind-
ly.” Let us use the TRS-80 Level Il
tape format as an example. The
cassette input port terminates within
the TRS-80 with a resistance of 100
ohms. A signal from the cassette with
a peak level of about 2 V is needed to
insure a correct load. If the cassette
record/playback head is correctly
aligned with the tape, and the signal
is adjusted (via the volume control
and our peak-signal-strength meter)
to a peak level of 2 V, then the
TRS-80 (or whatever computer you
have) should load correctly every
time.
Figure 1 presents the circuit for a
peak-signal-strength meter. The
signal from the cassette recorder
comes in jack 1 and goes out jack 2 to
the computer. Two halves of an
LM358N dual operational-amplifier
device are used to create a circuit that
is highly sensitive to voltage changes
in the 2 V region.
Although component layout is not
critical, a full-size, printed-circuit-
board pattern for this circuit is given
in figure 2. A 9 V transistor radio bat-
tery will have a life of around 2000
hours of continuous use. The unit can
be calibrated by marking the milli-
ammeter dial while applying a known
voltage from a DC 1.5 V flashlight
battery cell; the reading should not
change significantly when the polari-
ty of the input voltage is reversed.
The circuit is reasonably accurate in
measuring peak voltages of signals
with a frequency of up to 20 kHz, and
it will "then give good accurate
readings as long as the 9 V battery
supplies 5 V or greater.
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Figure 1: Schematic diagram of peak-signal-strength meter. This meter enables the user to present the cassette interface with a signal
of known peak intensity—usually, about 2 V. The circuit is designed to be sensitive to voltage changes around the 2 V area,
14 May 1980 © BYTE Publications Inc
Circle 6 on inquiry card. —
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Checking the Recorder Azimuth
If your tape recorder has an
azimuth-adjust screw, adjusting the
azimuth angle is a simple procedure.
You must first place in the cassette
recorder an azimuth-calibration tape
(see text box) or a similar cassette
tape recorded on a machine known to
be properly aligned. Then, playing
the cassette and monitoring the
recorder output with the peak-signal-
strength meter, turn the azimuth-
adjust screw until the meter reaches
its maximum reading. The reading
drops off on both sides of the optimal
position,
The meter can also be used to get
the best reading from a tape that was
produced on a tape recorder with
faulty head alignment. Simply
monitor that tape with the peak-
signal-strength meter, adjusting the
azimuth-adjust screw until the
recorder gives the strongest reading,
and use the recorder to load and
verify the tape. Once this has been
done, the recorder can be realigned
and a new tape can be made that you
can later load without the same kinds
of adjustment.
One method of improving
the reliability of cassette
tapes is to modify the
signal coming from the
cassette recorder.
Problems with Reading Tapes
With most computers, you will
need to load a tape using an input
peak-signal level of about 2 V (which
will appear as about half-scale on the
milliammeter of the peak-signal-
strength meter), With only slight var-
iations due to a particular comput-
er/recorder combination, the same
reading from the peak-signal-strength
meter will result in effective loads. A
cassette tape coming from a recorder
with a misaligned head will give a
lower reading than a correctly record-
ed tape for the same volume
setting.First try to load the tape after
increasing the recorder volume until
the peak-signal-strength meter gives
the customary peak reading. If this
does not work, you will have to load
the tape after adjusting the azimuth in
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16 May 1980 © BYTE Publications Inc
Circle 6 on inquiry card.
the manner previously described.
Whenever the tape head is mis-
aligned with respect to the tape path,
the peak-signal intensity will flutter,
even if the tape being played was
recorded correctly. This effect is
called skew, If the signal variation is
severe enough, you will be unable to
load the tape properly due to data
dropout, Signal flutter due to skew is
a subtle problem; it will not show on
a meter because no meter needle can
move fast enough to follow the flut-
ter.
Flutter can also be caused by tape
weave, which has a variety of causes.
If the pressure pad opposite the
record/playback head is not posi-
tioned properly, it will tend to push
the tape away from the center of the
head. This is aggravated by the fact
that most cassette recorders do not
maintain tension on the supply reel,
allowing the pressure pad to pull out
tape freely and push the tape away
from the center line of the head. Also,
a tape with a thin backing is more
susceptible to tape weave.
Altering Tape Waveforms
Another method of improving the
reliability of cassette tapes is to
modify (and sometimes rerecord) the
signal coming from the cassette
recorder. For example, several
waveform-changing interfaces that
improve the loading reliability of the
cassette are available for the Radio
Figure 2: Full-sized, printed-circuit-board
pattern for the peak-signal-strength meter
circuit of figure 1.
There is only one
high performance
VLSI computer
solution.
Intel delivers it.
Tools to solve the
ASM86
PL-M/86
PASCAL
RMX/86™
COBOL
FORTRAN
How Intel
delivers the key
to productivity
in the 80s.
As we move into the ’80s, the
increasing demand for complex com-
puter programs, the critical shortage
of programmers, and the seemingly
unstoppable rise in software develop-
ment costs will reach crisis proportions.
To understand how to bring this
situation under control, we have to
understand its cause.
In the 1970s, the microcomputer
was used successfully to lower the cost
of hardware engineering. Each new
microcomputer generation integrated
more and more of the system, lowering
the cost of design and making it easier
to put electronic intelligence anywhere
and everywhere. As hardware cost
dropped, rising software costs became
increasingly visible.
So, today, as costs climb, manage-
ment puts everyone under increasing
pressure to deliver projects on time and
on budget. Yet, the cost of programming
is still outpacing productivity. Software
development and integration still
lag the system hardware. The software
crisis of the 80s rages on.
Tools for structured solutions
Once a problem grows beyond a
certain point, the most efficient way to
salve it is with a top down approach,
You break the problem into units,
program and debug each one, and com-
bine the units into a unified solution.
That's the concept. But you can’t
stop there. In the '80s, bridging the gap
between a conceptual solution and a
working one will require toals as
efficient as the cop down method. New
tools, like a CPU with a dramatically
different architecture. An architecture
uniquely suited to a world of higher level
languages and structured programming.
Tools like a modular operating system,
of a kind never before available on a
16-bit microprocessor. Tools like the
software Crisis.
only complete family of programming
languages, because no one language
is right all the time.
Different languages have different
strengths and weaknesses, and using the
right language for the right jobcan
make your programming
upon the features you select, you save
from two to forty man years of program-
ming effort. That's an additional
two to forty man years you can devote
to your application.
Tools for realizing your
solution
easier. So, Intel delivers Peripheral
ASM86 Macro As- Controllers Of course, having
sembly Language the foundation and
for space and Co- the concept of your
speed sensitive processors solution doesn’t
modules. Our help if you can’t
PL-M/86 sys- write the pro-
tems program- grams to imple-
ming language ment it. So, Intel
and PASCAL Software delivers develop-
supportstructured Software Develoostect ment tools to sup-
: systems
programming at port you through
the systems and ap-
plications levels. FOR-
TRAN and COBOL
will also be available
With Intel's re-
the entire develop-
menteycle. Support
from source entry, with
CREDIT, a CRT based text
editor, through com-
location and link-
age tools, modules
Intel Structured System
pilingand debugging,
with an Intellec®
written in the
different languages are combined, with
library utilities and operating system
routines, into one, complete solution,
automatically. Using this modular
approach, and the right language for
the right job, your finished product is
clean, reliable, maintainable, and
understandable.
The critical module
Since complex software requires
sophisticated operating systems support,
the operating system is the most critical
module in your solution. It is the foun-
dation upon which your application is
built. It is also available, off the shelf,
from Intel.
Today, Intel delivers the RMX/86™
operating system. RMX/86 is new, and it’s
the first modular, real-time, multitasking
operating system for 16-bit microcom-
puters. File manipulation, task schedul-
ing, and interrupt control are configured by
you, according to the needs of your appli-
cation. There's no unnecessary burden.
Incel’s investment in the development
of RMX/86 is substantial. Depending
development system
and ICE™ hardware/software debugging
system. Intel's tools work with you. They
shorten development time and support
the structured approach you've taken.
Bur debugging software on a
development system is not the same as
testing it on the actual hardware.
The ICE modules help here, too.
During development, these tools let
you trace through your software and
debug it, symbolically, at the source
language level. Now, these In-Circuit
Emulators replace your prototype
hardware’s CPU to speed hardware/
software integration.
If your hardware is built from
components, ICE Modules will help you
separate the hardware and software
bugs, so you don’t spend your time fixing
engineering problems. If your hardware
is built around an Intel iSBC 86/12A™
Single Board Computer, you'll already
have a known, working hardware
environment for program testing.
You can use ICE Modules to concen-
trate your efforts on debugging your
software.
Either way, the same software,
operating system support and debugging
tools are available to help you bring
your application to life.
Synergy for high performance
In the ‘60s and '70s, programs were
used to instruct computers. Applications
of the '80s require programs to be the
solutions to problems. High perfor-
mance solutions will be the result of
synergy between the hardware and
the software.
To create this synergy today, Intel
delivers the 8086 processor. The 8086
processor is unique. Instead of a linear,
or flat architecture, the 8086 is the
only microprocessor optimized to work
with high level languages and the
structured solutions they implement.
For the specialized needs of the ’80s,
the 8086/87 and 8086/89 co-processing
systems will set the standard of per-
formance for mathematical processing
and I/O bound applications.
And Intel peripheral controllers
contribute to your system throughput
by freeing processors for more
computation.
Your software design may be
revolutionary. And with help from an
architecture designed to support your
structured solution, its performance
can be revolutionary, too.
Intel's software tools let you con-
centrate your planning on the payoff—
getting to market today with a superior
product. To take your first step to
higher productivity, fill out and mail
the coupon on the other side of this page.
It's a productive use of your time.
Europe: Inte! international, Brussels,
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Your first step toward productivity
tion requested below and send scissors handy, give us a call
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solving the software Santa Clara, CA 95051. Indicate material out to you. Or call
crits of the"BOn, SRAM meric
To find out more about our make sure you get the appropriate _ the software crisis of the ‘80s.
solutions, fill out the informa- material. If you don’t have a pair of It all starts right here.
Name.
Title
Company
Division. 2 ss ——
Address
City, State, ZIP
—— Ihave an immediate requirement, please telephone me at ( )
—— I need additional information.
Please put the letter corresponding to your yearly requirements in the line to the left of those
products of interest.
W for 1-10 per year
X for 11-99 per year
Y for 99-999 per year
Z for over 1,000 per year
__ A 16-bit Microprocessors ___ F EPROMS __L Macroassembler
_— B 8-bit Microprocessors _— G Bubbles __. M Operating Systems
__ C Single-Chip —— H Single Board Computers (RMX/86™)
Microcontrollers __ 1 Development Systems __N Telecom Products
__ D Peripheral Controllers __ J Debug Tools ——O Military Products
—E RAMS ___ K High-Level Languages — P Workshops
(—_ ae )
‘
012345 Circle No. 426 for information.
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See coupon below for ordering.
Shack TRS-80. About two winters
ago here at Cook Laboratories, we
developed a modified tape format
that records more reliably on the
TRS-80. Without going into the
details of the TRS-80 tape format, I
can say that the unaltered tape signal
crowds too much information into a
given space and thus opens itself to
reliability as well as saturation prob-
lems. The latter problem is what ©
makes the TRS-80 normally so vol-
ume sensitive. The waveform we use
at Cook Laboratories, when record-
ing tapes for the TRS-80, reorders the
waveform shape and narrows the
pulse width so that the cassette inter-
face does not get confused.
The various waveform modifica-
tions could certainly be used to im-
prove the reliability of the cassette
storage on other microcomputers. For
example, on the old-model PETs,
there is no way to alter the volume
level of the built-in cassette recorder.
However, Commodore can provide a
documented program called $S-21.
This program, when running,
monitors tape being played in the
PET cassette deck and displays cer-
tain information about the quality of
the tape signal on the PET screen.
This is a very effective program to
have if you know how to use it; Com-
modore is the only manufacturer I
know that supplies a program like it.
Tape Is Also a Factor
Several factors having to do with
the cassette tape itself can also affect
the reliability of tape loading. As I
mentioned before, a tape that is too
thin will likely give in to tape weave,
Tape stiffness is a property of the
thickness of the backing and is pro-
portional to the third power of the
gauge thickness of the backing. This
indicates that you should not use
long-playing cassettes for program
and data storage.
The thickness of the magnetic
coating affects the reliability of
cassette storage, but in a different
way. Standard audio tapes, chrom-
ium dioxide or otherwise, are not op-
timal for digital recording because
they are designed to give good fre-
quency response in the low frequen-
cies. But low frequencies are not
needed here; rather, well-defined and
sharp waveform transitions are what
count. A thinner magnetic coating
than what is used in standard audio
cassettes results in nice improvements
both in waveform resolution and
sharpness of transition. Not inciden-
tally, Cook Laboratories markets a
custom line of digital cassettes under
the trademark MICROFUSION. This
tape has a thinner chromium dioxide
coating and a heavier and, therefore,
stiffer backing, both of which make it
well suited for digital storage.
Cassette tapes can be used for
reliable mass storage if the tape
recorder is kept clean and properly
aligned, if quality tape (especially
tape made for digital storage) is used,
and if the signal going from the
cassette to the computer is monitored
and kept constant (from tape to tape)
with a peak-signal-strength meter,
Although disks are readily available
and bubble memories are not far
away, no medium will ever become
obsolete as long as it provides a need-
ed function, Cassettes, too, are here
to stay. m
The following items are available
from Cook Laboratories, 375 Ely
Ave, Norwalk CT 06854:
AZ-80 Precision azimuth cas-
sette, chromium dioxide.
$14.95
Kit for peak-signal-
strength meter, including
board, litho panel, screw-
driver, meter, case, and
instructions, less battery
$25.90
Printed-circuit board for
peak-signal-strength
meter, etched and drilled
PK-80
AZ-B1
$2.50
MICROFUSION digital
cassette, C-10 $3.26
C-20 $3.68
C-30 $4.09
Add $2.00 for each order for
handling and shipping within the
continental United States. Connec-
Heut residents add 7% tax.
DO YOU SEE EYE TO EYE WITH YOUR APPLE?
The DS-65 Diglsactor® opens up a whole new world for your Apple Il: Your computer can now be s pat of the action, taking plotures to amuse your
ing your house while you're away, taking computer portratts the applications i
It converts a TV camera's output into digital informat
* High resolution: 256 X 256 picture slement scan
friends, watct
* Precision: 64 levels of grey scale
‘* Versatility: Accepts either interlaced (NTSC) or Industrial video input
* Economy: A professional tool priced for the hobbyist
The OS-65 Is an intelligent peripheral card with on-board software in 2708 EPROM.
‘Check these software features:
* Full scre
digitizing by Basic pi
= Utility funct
Let your Apple see the world!
Pets help $340.95
Video FSI| Camera Price $299.00
SPECIAL IAL COMBINATION PRICE: $599.00
“KIGRO
Fol piracy to Apple Hi-Res Lineal
ited fand program
© Line-scan diattzing Yar veading charts or vecking objects
jons for clearing and copying the Hi-I
jas screen
abound! The OS-6
ion that your compuiter can process. The DS-65 features:
js a random access video digitizer.
APPLE SELF-PORTRAIT
LWoRKS P.O. BOX 1110 DEL MAR, CA 92014 ay
18 May 1980 © BYTE Publications Inc
Circle 7 on inquiry card.
Circle 8 on inquiry card.
PROGRAM EPROMS
WITH YOUR APPLE
COTTE?
A DC
-to-DC
Converter
Michael Picco
POB 516
Corte Madera CA 94925
Have you ever had the need for a
bipolar power source, but had only a
positive voltage available? With the
help of a timer and a few external
components, this problem can be
overcome. The circuit in figure 1 is
sufficient for powering op amps or
similar devices requiring a supply
current of 25 mA or less at —12 V.
The heart of this circuit is a 555
timer that provides a drive signal of
approximately 20 kHz to a voltage
doubler, This signal is removed if the
+12v
magnitude of the output voltage ex-
ceeds that of the supply voltage. In
this sense, the converter operates as a
switching-mode regulator,
The output voltage is set by con-
trolling the timer via the reset input
(ie: pin 4). When the output voltage
reaches a negative potential with the
same magnitude as the supply volt-
age, a low logic state is placed on the
reset input, causing the timer output
to go low and the increase in voltage
magnitude to cease.@
Re AL RS 4 oF
10K 2.2K 56K 56K >
7 4
DISCHARGE RESET 3
6 20pF DS
THRESHOLD 3 15¥ insis
OUTPUT
, ourPuT SVG aad
2) rriccer tases c2
tL 20pF
15y ¥ Thos Yoour
cl D2 15V
0.001 mF 1N918
Number Type GND +12V
1 55 1B
ca
Le 20uF
+] 1sv
Figure 1: Schematic for the DC-to-DC converter. The 555 timer produces a rectangular
wave at about 20 kHz, which is inverted by the diode-capacitor voltage-doubler
arrangement. A feedback signal reaching the reset pin of the 555 regulates the
magnitude of the output, which is —12 V at 25 mA.
20 May 1980 © BYTE Publications inc
Circle 9 on inquiry card. —,-
Agrowin
firs of tous to
expand the Apple.
74404 Programmabie Interrupt Timer Module,
Time events in four operating modes —continu-
ous, single shot, frequency comparison, anc
pulse width comparison. Includes three 16-bit
interval timers, plus flexible patch area for
external interface. Programmable interrupts,
on-board ROM, and much more.
77204 Parallel interface. Two bi-directional 8-bit
V/O ports will connect your Apple toa variety of
parallel devices, including printers, ee tape
7811B Arithmetic Processor, Interfaces with
Applesoft, so you just plug in and run. Based
on the AM 9511 device, provides full 16/32-bit
arithmetic, floating point, trigonometric, loga-
rithmic, exponential functions. Programmed I/O
data transfer, much, much more.
TNA, Serial interface. Conform-
ing to RS-232-C A thru E 1978 standard, this
card will drive a variety of serial devices such as
CRT terminals, printers, paper tape devices, or
communicate with any standard RS-232 device,
including other computers. Full hand-shaking,
and fully compatible with Apple PASCAL!
7470A 3% BOD A/D Converter, Converts a DC
voltage to a BCD number for computerized
monitoring and analysis. Typical inputs include
DC inputs from temperature or pressure
transducers. Single channel A/D, 400 ms
per conversion.
‘7490 GPIB IEEE 488 Interface. A true imple-
mentation of the IEEE 488 standard—the
standard protocol for instrumentation and test
devices. Control and monitor test instruments
‘such as digital voltmeters, plotters, function
generators, or any other device using the
IEEE 488.
‘714A PROM Module. Permits the addition to or
replacement of Apple II firmware without
removing the Apple If ROMs. Available with
on-board enable/disable toggle switch.
7800 A Wire Wrap Board. For prototyping your
own designs.
* 7510A Solder Board.
78904 Extender Boord,
016A 16K Dynamic Memory Add-On,
Watch this space for new CCS products for
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e
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like function generators or plotters.
Apple has built a great computer. We at CCS have And we have tools to connect the Apple to the outside
built a great line of peripherals and components toexpand _ world, including A/D converters and interval timers with
the Apple. To do almost anything you want to get done external interface.
with a computer. We make components for the S-100 bus, the PET, and
If you want to do business with an Apple, we've got the TRS-80, too. We built our products to deliver hard-
tools to connect the Apple to standard business printers and nosed value to the OEM, and to the inventor who knows the
terminals. Or to modems, for communications over tele- best, at prices that are unbeaten.
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If you want to apply your Apple to engineering, scien- _ see things our way. Because for serious users with serious
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California Computer Systems
] 250 Caribbean Sunnyvale,CA94086 (408) 734-5811
Cisrcia’s Circuit Cellar
Copyright © 1960 by Steven A Ciarola. Ail rights msprved,
I/O Expansion for the Radio
Shack TRS-80
Part 1: Principles of Parallel Ports
I receive a lot of mail: enough that
I'm beginning to feel like the “Dear
Abby” of the personal computer
ranks. The sources of the letters range
from high school students asking for
advice on science fair projects to ma-
jor corporations seeking consultant
services. Even though it takes con-
siderable time to answer this mail, I
regard it as a significant opportunity
to gauge reader interest. Every letter
in some way contributes to my choice
of article topics, either through sug-
gestions or by continued occurrence
of similar questions.
Recently, my mail has been
dominated by owners of the Radio
Shack TRS-80 Model I thirsting for
hardware expansion by means other
than Tandy Corporation equipment.
The majority of questions concern
connection of my interfaces to the
TRS-80 expansion connector.
In general, I have tried to present
projects that are computer indepen-
dent. That is, the interfaces described
are driven through parallel input/
output (I/O) ports rather than direct-
ly from a computer bus. This had not
been a problem in the past, because
virtually all of the early personal
computers incorporated some parallel
I/O capability. For those exper-
imenters interested in enhanced 1/O
capabilities, I presented the article
22 May 1980 © BYTE Publications Ine
Steve Ciarcia
POB 582
Glastonbury CT 06033
“Memory-Mapped I/O” in the
November 1977 BYTE on page 10 (re-
printed in Ciarcia’s Circuit Cellar
Volume I, BYTE Books), which
detailed parallel-port construction.
In the 2% years since that article
was first published, a number of
A port is a hardware
channel for the computer
to transmit and to receive
data via an external
peripheral device.
significant changes have occurred in
personal computing. Most important-
ly, the Radio Shack TRS-80, the Ap-
ple Il and the Commodore PET were
introduced. The difficulty in main-
taining and operating a computer is
no longer a serious consideration for
most computer enthusiasts. Much of
my mail indicates that a new explana-
tion of parallel and serial I/O is in
order, and that it is time for
hardware-expansion circuits to be
detailed.
This month's Ciarcia’s Circuit
Cellar is the first of a two-part article
on serial and parallel 1/O port expan-
sion of the TRS-80. The first part em-
phasizes parallel 1/O, and the second
part is concerned with serial interfac-
ing. The result will be a complete
Radio Shack software-compatible
communications interface capable of
supporting a variety of serial- and
parallel-interfaced peripheral devices.
The hardware was designed and the
components were selected to be
economical to build and easy to check
out. First, here is a brief review of the
basics.
What Is an I/O Port?
Just as some people are initially
confused with the terms hardware
and software, some find the concept
of input and output ports difficult to
understand without substantial ex-
planation. The classical definition: a
port is a hardware channel for the
computer to transmit and receive
data via an external peripheral
device. The key words in this defini-
tion are external and data which imp-
ly externally collected information;
the channel through which this data is
obtained is called a port. A printer is
a typical external peripheral device.
The characters to be printed are sent
from the computer to the printer. In
some of the more sophisticated units,
status signals such as busy and out of
paper are returned to the computer
from the printer.
Ports can be either parallel or
seria], In parallel mode, data is
transferred in increments equivalent
to the word size of the computer. On
the Z80, for instance, an 8-bit
microprocessor, an output instruc-
tion through a parallel port transfers
8 bits at a time. A 16-bit processor
such as the Intel 8086 transfers data in
16-bit increments. The number of bits
transmitted simultaneously by a
parallel port is dependent upon the
size of the microprocessor data bus
and how many bits the processor can
transfer simultaneously.
However, serial data is always
transmitted a single bit at a time, ac-
cording to a fixed schedule defined by
the data rate (usually expressed in bits
per second, or bps) and a few specific
options. The microprocessor has no
single instruction that transmits serial
data, It must rely on another device
called a universal asynchronous
receiver/transmitter (UART) to put
the data word into serial form and
transmit it. Any communication be-
tween the processor and the UART is
in parallel form and is done through
the processor’s memory reference or
1/O data-transfer instructions. A
more in-depth discussion of serial
ports will be presented next month in
Part 2.
Address, Data, and Control Buses
Consider a computer system that
includes a printer, video terminal
with keyboard, and an audio cassette
recorder as peripherals. Data would
have to be relayed to the printer, to
and from the video terminal, and to
and from the cassette recorder. How
can the computer tell the difference
between data destined for the ter-
minal and the data destined for the
printer?
Most microprocessors incorporate
a bidirectional data bus and an ad-
dress bus: this is shown in figure 1.
To keep track of the data transfer be-
tween the processor and its peripher-
als, the system uses a quantity of con-
trol signals which together can be
called the contro! bus. The usual 8-bit
processor has an 8-bit data bus, a
16-bit address bus, and a dozen or so
control signals.
When the microprocessor is read-
ing a data byte from memory, the ad-
dress of the memory location being
referenced is placed on the address
bus. Memory information stored at
that location goes on the data bus and
flows from memory to the processor,
When data is being written into
memory, the operation is reversed. A
16-bit address bus allows the pro-
cessor to directly address 65,532 (ie:
64 K) memory locations.
/)
iiffak
In an 8080 or Z80 processor there is
a specific set of instructions that per-
form input/output functions. The
operation of these I/O instructions is
similar to that of memory-reference
instructions, except that only 8 bits of
the address bus are used. These 8 bits
Photo 1: There are a variety of ways to decode the address for a particular input/output
(I/O) port from the signals present on the address bus. The least expensive method uses
inverters and printed-circuit-board jumpers to select the correct logic polarities. Three
address lines are connected through each 7404 hex inverter with two possible connec-
tions for each address line. A connection to the upper trace on the circuit board decodes
a logic |; a connection to the lower trace decodes a logic 0.
a!
Photo 2: A more expensive and more easily changed addressing scheme employs dual-
in-line-pin (DIP) switches and exclusive-NOR gates. The schematic diagram for this is
shown in figure 3b.
‘May 1980 © BYTE Publications Inc 23
Circle 10 on inquiry card.
Have some
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memories.
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I PROM card has 2708-type memory
8 Quality board construction ll 0-4 wait states
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le 5 MHz RAM boards.
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Call or write Artec for details
SIU!
EEL
ARTEC ELECTRONICS, INC.
605 Old County Rd., San Carlos, CA 94070
‘Telephone (415) 592-2740
24 =~ May 1980 © BYTE Publications Inc
Photo 3: Prototype of an 8-bit I/O port for the Radio Shack TRS-80. The ribbon cable
at left connects to the expansion port on the keyboard/processor unit. The two 1/0
ports are brought out to the ribbon-cable connector on the right edge of the board.
designate one of 256 possible 1/O
ports. In the case of the example
system, a separate port address
would be used for each peripheral.
Keeping track of bus direction and
information flow is a matter of pro-
perly decoding the control signals
during program execution. In a Z80
for instance, any memory-reference
operation is signified by the control
signal MREQ in the processor going
to a logic 0, or low, state. An input or
output operation is designated by the
I/OREQ control signal being at
logic 0.
The direction of the data bus
depends on whether the processor is
trying to read or to write data. If the
processor is in a read mode, the RD
control signal becomes a logic 0; if the
processor is writing, the WR line is
in the O state. Monitoring these four
lines, MREQ , I/OREQ, RD, and
‘WR, gives us all the information
necessary to support I/O decoding
functions. Figure 2 demonstrates how
these control outputs are combined
for system use.
Address Decoding
So far we have discussed how to
determine when the processor wants
to send a character to an output
device. In such_an operation the
T/OREQ and WR lines are both
low. To tell the difference between
data for the printer and data for the
terminal, we must decode the 8-bit
port address.
The port address is determined by
the logic voltages present on the low-
order eight lines (that is, the 8 least
significant bits) of the address bus
during I/O operations, Various
techniques can be employed to de-
code these lines. Figure 3 outlines a
few simple methods, The objective,
whatever the logic employed, is to
produce a single pulse (ie; a strobe)
whenever the logic states representing
a particular address appear on the ad-
dress bus. To eliminate false outputs
when the processor is executing in-
structions not dealing with I/O, it is
best to combine control and address
signals as demonstrated in figure 4.
If you own a 6800- or 6502-based
system, you have probably noticed
that the processor has no special 1/O
instructions. This does not mean that
these processors have no external
communications capability, only that
these processors communicate with
peripheral devices differently. How
can we discover this different
method? Let us begin by looking
closely at the 1/O functions of the
8080 and Z80 that we have just
discussed.
A close inspection of the I/O func-
tions of an 8080 or Z80 should point
Text continued on page 30
Circle 11 on Inquiry card. —
ar E r pe
HodIDDIN. ooay
PSSTPUFHES*atnaswine WRMRTIaZ 2%
BdG HS fpage
VIDEO DATA PROCESSOR
Called the VDP, comes complete
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SYMTEC X-10 CONTROL
Now for the Apple II, a low cost
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Muse’s Pilot Il Language using the
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Circle 12 on inquiry card.
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DISPLAY/EDIT TERMINAL
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STANDARD FEATURES (partial list)
= Mi ‘controtled
ra) PS292C and 20 ma current oop
{0 baud rates—-T5 to 19200
{LEE tea
12 x 10 character resolution Figure 1: Block ram of a microcomputer system that uses an 8-bit microprocessor
ual intensity display 4 . “
© Clear full intensity data oniy such as the Z80. This system uses bussing techniques that are both multiplexed and
te dlr bidirectional.
280 SIGNALS CONTROL STROBES
&
yaisoo |
delete 1 4
Sa |
1 Airareea, we OLDS, _1y—~, |
o anasto ures. including curkor controls OE
remote commands such as con a alle; [ +} ions READ (RO™*)
|
spaces, end of ine, end of screen: Sol hi,Jo,20r0
2
intensity: set blink; etc
© Opti
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h
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lonal scraen print & 2nd page memory i |
fone escatanl praca spaatins |
the Computex X-820 le unmatched in its class, Tew >t>s 1a ‘ *
] i [se 170 WRITE (0UT™?)
Allfeatures of the Hazsiting 1400 and ADMGA i | ; |
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Ted matin Re I |
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= ... dackspace. = 5 9
TTabbing... iD olf >° —s
samen pad. | i lle } i 170 READ (In*)
| | i. |
$799 | |
i |
= Loi Lis,
wies9956 |] we CLS Vaal 2 SL meee WE vs
PERKIN-ELMER (Model 550) ar 4 |
er
rere om eeroe a
Soe nooner Figure 2: Control signals on the Z80 microprocessor. The Z80 uses a variety of con:
within 10 days. X-0000 CPU $2995. Pascal signals to keep data flowing at the right time and in the right direction. Four control
pa ec el hater kann signals are as follows: the MREQ line goes to a low state (ie: a logic 0) when a
Sou down guarantees framy. recent ade. reference operation is in progress; the I7OREQ_ line goes to a low state when
jown guarantees prionly IL residents a memory- reference ion 5 ie]
ala eee an input/output (I/O) operation is in progress; the RD line goes low when the pro-
Chul woaca wine ee eneee cessor is reading data from memory or from a peripheral device; the WR line goes low
Nationwide service contracts. Custom software. when the processor is writing data to memory or to a peripheral device. The RD and
‘We provide expert technical support. WR
WR signals control the direction that data flows along the bidirectional data bus.
Monitoring these four lines gives us all the information necessary to support I/O
(312) 684-3183 decoding functions. :
COMPUTEX
Signals from the four processor control lines are logically combined to form control-
strobe signals that perform specific functions. The characters in parentheses give the
“The Computer Experts” names by which the control-strobe signals are known in the documentation for the
8710 Drexel Avenue i
‘Chicago, IL 60637 Radio Shack TRS-80.
26 May 1980 © BYTE Publications Inc Circle 13 on inquiry card. —>
eit tabs Fan
— Blinking/blank fields
Upperfower
case char.
ed fields
Underiining
re sereen
#2x 1char. res.
Blinking cursor
* 9 Baud rates
{75-9600 Bud)
* Self test
* Auxiliary port
Function/edit keys
'ypewriter/TTY
—— keyboards
Numeric pad
Model 9200 shown,
~~
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COMPARE PRICE.
ADDRESS
BUS BINARY WEIGHTING on PORT HARDWARE DECODER
SIGNAL CIRCUIT FOR THAT CODE
N+0 OR 1 LOGIC LEVEL *
A? —» 27x N= 128X1» 128 ———
AG ——» 25xN+ 64X1* 64 c—_—
45 ——» 28xn+ a2x0*8 0 —>—
Aa ——* 24x 16x0% 0 ro—p—
DEC
ADDI
A3——+ 23xN+ oxo + 0 oe Pee ae
A2——» 22xNe x1 © 6 ——
Al——+ 2lxns 2x0 + 0 lc
i>
¢
PORT ADDRESS
C5y5
am
oo
S
as
AO ——» 20XN=+ 1X1 © 1
Figure 3a: Various methods can be employed to decode the address signals that appear on the address bus during 1/O operations.
Here, various inverters and an eight-input NAND gate are hardwired in a configuration that will produce a logic 0 output for one of
256 possible I/O port addresses. The logic 0 output can be used to activate the interface for the peripheral device. Here the circuit
decodes the address hexadecimal C5, or decimal 197.
toa
7aisos
iN
DECODED ADDRESS
+5V Bia
1K
TYPICAL FOR 4
ADDRESS BUS INPUT i
our
arc a ee a oo SHB
7alsas
wo al orf 2 SUZ
ast > 134a2 a2e4 6. oo SHS.
uc 1h Bap oo SMS
+5V
wk
6 TYPICAL FOR 4
IT
a3C> 10} a @ ee-—a oo SMA
TALSB5
a2 12},, Py tee oo SH
ac 13] 42 e2p4 eo 52
40> A5ha3 ea a SWI
Figure 3b: Another method of decoding an address signal. Two 4-bit comparators can be cascaded together to decode an 8-bit ad-
dress. The desired 8-bit port address is set up on switches SW8 thru SW1. When the combination of high and low logic states that cor-
responds to the desired address appears on the address bus, the output signal produced at pin 2 of IC3 (the 74LS04 inverter) will go
low to a logic 0 state. This decoding method allows the port addresses to be easily changed, but the method here is considerably more
expensive than the decoding method shown in figure 3a. The switches are single-pole, single-throw (SPST) types; an open switch
shows logic 1, and a closed switch shows logic 0.
28 May 1980 © BYTE Publications Inc Circle 14 on inquiry card. —>
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NorthWord is the central building
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Text continued from page 24:
out that the I/O instructions bear a
surprising resemblance to memory-
reference instructions, The 6800 and
6502 microprocessors actually
allocate a certain portion of their
memory address space to be decoded
and to function as 1/O ports,
This technique, which can be used
on the Z80 and 8080 just as easily, has
certain advantages in speed and ease
Number = 7}
{C1 7AtS266
74LS266
74LS30
of use over direct I/O instruc-
tions. This technique is referred to as
memory-mapped I/O, An illustration
of the logic associated with this
method is in figure 5, For a more
rigorous analysis of memory-mapped
I/O, I refer you to the November
1997 “Ciarcia's Circuit Cellar” article
previously mentioned.
The final area for consideration is
the actual transfer of data to and
+5V¥ GND
14 7
+5V
tye ihe J
ADDRESS
Bus
103
74L$30
| stcooee ADDRESS
Ww
from the bidirectional data bus. The
circuits of figure 4 and figure 5 tell on-
ly when the I/O operation occurs.
Additional logic has to be provided to
place data on the bus during an input
instruction or to latch and hold the
contents of the data bus during out-
put instructions,
When the 8080 or Z80 assembly
language instruction OUT (N),D is
executed, the contents of the ac-
cumulator, D, are placed on the data
bus and written into device N, The
same is true for the BASIC-language
instruction OUT N,D. The data is ac-
tually valid during only a few clock
cycles, perhaps 500 ns. Making this
data available for longer periods of
time requires the addition of an 8-bit
latch: the latch is made from a set of
clocked flip-flops.
The output lines are attached to the
data bus, When the proper output in-
struction is executed, signified by a
strobe signal from our address and
1/O WRITE decoder circuit as
shown in figure 6, the contents of the
data bus are transferred into the 8-bit
register in synchronization with the
processor clock signal. This combina-
tion of circuitry is commonly called
an 8-bit latched parallel output port.
External devices cannot be directly
connected to the data bus for input,
because of the possibility that in-
terference and bus-loading problems
will result. A three-state buffer is used
as a gate to allow signals from the
peripheral device to be placed onto
the bus at the appropriate time.
During an input operation the pro-
cess used for output is reversed.
When the proper input sequence is ex-
ecuted, signified by the appropriate
output from the address decoder and
I/O READ decoder, the 8-bit three-
state buffer is strobed into operation
during the few clock cycles it takes
for the processor to execute the input
instruction. Logic levels present on
the buffer input lines during that in-
stant become impressed onto the data
bus and are transferred into the ac-
curnulator. Figure 6 shows the logic
elements that perform these func-
tions.
Add Parallel 1/O to Your TRS-80
I have been told that the TRS-80
Figure 3c: Another method of decoding an 8-bit address, using exclusive-NOR gates and Model | is currently the largest-selling
an eight-input NAND gate. As in figure 3b, the desired port address is set up on switch- personal computer. Unfortunately it
es SW8 thru SW1.
30 May 1980 © BYTE Publications ine
Text continued on page 38
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‘Apple Ilis a trademark of Apple Computer Inc.
3TRS-80is a trademark of Radio Shack, a division of Tandy Comp.
chusetts, Alaska, and Hawaii).
Integral Data Systems, 14 Tech Circle,
Natick, MA 01760. (617) 237-7610.
Circle 15 on Inquiry card,
Integral Data Systems, Inc.
Circle 16 on inquiry card.
GET IT
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32 May 1980 © BYTE Publications Inc
our*
rowan: O—————————____——_
STROBE
as C>————_-
ars >—— | nor use ror
Ala Ss 1/0 APPLICATIONS
ar OC
a c—-
ao L>——_
a oO ADDRESS DECODING
eL ‘SET FOR PORT 7
» >—Po
aS
TALS30
1
Cc > 4L$32 uw
eC > 3 i DECODED
a O—
CS >
ao L>————_]
@ BIT LATCHED PARALLEL
OUTPUT PORT
annnaaal
8 BIT REGISTER
LATCH O11 THRU DIB
OUTPUT
‘STROBE
Figure 4a: Block diagram of a typical parallel output port. The logic that decodes the
8-bit port address was shown in three forms in figure 3. The signal from the address-
decoding circuit is logically combined with one of the control signals from figure 2 (I/O
WRITE) to produce an output strobe signal that activates the 8-bit output latch
register.
as C————_
ais (>——— | nor useo ror
wat 170 APPLICATIONS
a2 C——_
ao
anv C>———_
al ADDRESS DECODING
ae (= FOR PORT 7
“>——_o
74Ls30
8 BIT STROBED PARALLEL
INPUT PORT
aaaaaael
8BIT
THREE- STATE BUFFER
STROBE DO] THRU DOS
at > . Ppa ar
a > DECODED
ao INPUT
STROBE
a >
ao >
in*
170 READ
STROBE
Figure 4b: Block diagram of a typical parallel input port. Note the resemblance to the
output port of figure 4a.
Circle 17 on Inquiry card. —
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7aisaz 3}
[20 JNOT USED
47010
OECODER 7/9 __— port FF
: 6+ {> port Fe
7a. s32
—— tour) 5}*>—_1-> oat Fp
To ware > 2
at sf} > ort Fe | 9 pecoped
ouTPUT
un*) e a+ > pont a f Staoses
70 READ L>——______ 14 3
a 8 2--——_{ort Fa
A 12> port Fs
of}. port 8
ao—————_ b
7ALs32 741842 }\woy usco
as 74.830 atte
) 470 10
asC> DECODER 7 }2 ——1—> port FF
a > f z
ADDRESS 6/-+——1_> port Fe
Bus as —_—"_ 1215 sfé [> Port Fo
5
13 4-——{>pont Fe a DECODED
az2C> ic PUT
14 3}*- > port Fe STROBES
a> td 3
oo
ae 5], 2 : PORT FA
1p-——1> port Fs
of} ort Fs.
Figure 4c: Schematic diagram of a circuit that produces eight decoded input-strobe signals and eight decoded output-strobe signals.
The port addresses produced are hexadecimal F8 thru FF. Circle 18 on Inquiry card.
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Circle 19 on inquiry card.
BYTE May 1900
35
twr*) TALSOS
mem waite [> [>
ays >]
au CH
ais >} za 30 74.820
———— a
ao PORT FFFE
a >——___—_——} OUTPUT STROBE
a —————
as OF
as CO
ao“
as O——_
ce ae 74.820
“© PORT FFFE
asc INPUT STROBE
loo
a —_
a0 L>—_ >
«Ro*)
WEM READ [>— -
Figure 5: Memory-mapped input and output, Some microprocessors do not have specific input and output instructions. In systems
that use such microprocessors, the I/O port hardware is wired as a memory location; 1/O operations take place using the memory-
reference instructions (eg: load-into-accumulator and store-in-memory instructions) of the microprocessor. This type of addressing is
called memory-mapped I/O, and all sixteen lines on the address bus must be decoded to perform an I/O operation.
LATCHED
PARALLEL OUT PARALLEL INPUT
87 86 B5 B4 B3 82 BI BO 87 86 85 B4 83 B2 Ol BO
is. 2 {5 |s |o fre fis fas fis 2 7 ja fas Je fra fe fu
DECODED 1 az] 19 22 30 40 Se 6e 79 89 Al A@ AZ A3 AB AZ AG AL
ouTeuT ck 1g
STROBE 1c. (c2 DECODED
74.8377 7ALS244 INPUT
Ue et? ar
on 10 2D 30 4D 50 60 70 aD vr Ya_ye ya Ya ¥2 Ya Yi
B 4 |r |e [is [is fv pe hays jes pss]? iz |e
8 BIT REGISTER BBIT THREE-STATE
BUFFER
o7 C> > 07
os C> + {> 06
os (> © {> 0s
pata { 04 L> est > oa
aus ) 03 [> + {> 03
oz > +. {> v2
ao +- + > 1
oo [> + {> vo
Figure 6: Data connections in input and output ports. Once the proper port address has been decoded and combined with the read- or
write-control signal to form an I/O strobe signal, the actual process of accessing the data bus for data transfer is relatively easy.
For input to the accumulator (the most common pathway for I/O), a three-state buffer is used in conjunction with the decoded
input-strobe signal that controls the enable line of the buffer.
For output from the accumulator, an 8-bit latch is connected to the data bus. During the execution of the output instruction, the
contents of the data bus are clocked into the latch register and are latched there by the output-strobe signal.
36 May 1980 © BYTE Publications Inc Cir
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