Alan Lehotsky wrote:

> Can someone concisely explain what the difference is?

> Can I use a drive that supports fast or wide or ultra with

> any old SCSI controller and just not get the maximal performance?

> Or, is it that the interfaces are incompatible?

> -- Al

This is probably explained better in the FAQ over on comp.periphs.scsi,

or that might be comp.periph.scsi. But here it goes:

[2012 Edit: comp.periphs.scsi FAQ, Part 1 of 2

[2012 Edit: comp.periphs.scsi FAQ, Part 2 of 2

In the beginning there was SCSI and it was pretty good and it maxed out

at transfer rates of about 3 MB/s. But that was the early 80's, so

it was faster than any hard drives worked anyway. There were

ambiguities in the IEEE standard as written, so SCSI from one company

wasn't necessarily compatible with SCSI from another company. This

problem mostly expressed itself in the PC world as little proprietary

SCSI controllers that were included with scanners or other peripherals

and that wouldn't work with any other SCSI devices. We had it good

in the Mac community when it came to SCSI, except that most of Apple's

implementations would do maybe 1.5 MB/s transfer rates.

SCSI was based on a 50 wire cable transferring 8 bits of data in

parallel. The other 42 wires are ground wires and various control

signals. The frequency of data over this cable is also part of

the standard but I don't remember the number. I think it was

10 MHz. [2012 Edit: It was 5 MHz.]

So it was decided to write a new SCSI standard that would vanquish the

ambiguity and while they were at it, they decided to add some

improvements, because 3 MB/s was getting pokey.

Thus was SCSI-2 born. By making use of synchronous transfers (or maybe

that was asynchronous transfer [2012 Edit: It was synchronous],

I get the two confused) SCSI-2 was

able to up the transfer rate to 5 MB/s using the same 8 bit parallelism

at the same frequency as the old SCSI standard. But the standards-makers

were not satisfied. So they added two more options to SCSI. It could

be Fast--operating at twice the normal frequency, and/or it could

be wide--transferring 16 bits of data in parallel instead of 8. And just

to help confuse things, they prescribed a new SCSI connector to replace

teh old Centronics 50--the High Density SCSI connector--often called

a SCSI-2 connector because it came out when the new standard was

written.

And the standards-makers declared backward compatibility. The original

SCSI devices would work on the same bus as the SCSI-2 devices. And Fast

SCSI-2 devices would work with them all. But to take advantage of the

new features, both of the devices communicating must have the same

features. What this means in proactice is that the user would never

get performance better than what his SCSI controller supported and the

controller would have to slow down to the speed of whatever device it

was talking with at the moment.

Because Wide devices transfer 16 bits at a time, they need a different

cable. You must have some extra wires to carry those 8 extra bits.

So Wide devices usually go on their own 68 wire bus. But those wily

SCSI Standards-writers thought of everything. If you put an

adapter on a wide device and attach it to a narrow bus, or put

an adapter on a narrow device and attach it to a wide bus, the

controller and the device will negotiate their transfer method,

and agree to just use 8 bit transfers, so that you don't have the

other 8 bits getting lost in the ether somewhere.

Wide devices usually cost a little extra, so people usually don't

buy them and then handicap them by putting them on a narrow bus.

And putting a narrow device on a wide bus can decrease the overall

performance of the wide bus, as well as causing issues with SCSI

ID's over 7 (wide uses ID's 0-15), so it's best to avoid that too,

if you can.

A note--wherever I write "wide" I really mean Fast & Wide. I've

never seen one, but a SCSI drive that was simply Wide, would have

a maximum transfer rate of 10 MB/s, the same as a Fast narrow one.

It's the combination of Fast (twice the bus frequency) with wide

(twice as many bits in parallel on the bus) that gets the

20 MB/s theoretical transfer rate of Fast & Wide.

Still later a newer SCSI standard was born and it was called

Ultra-SCSI. I'm less familiar with this new stuff, but somehow

it upped the maximum transfer rate on the old familiar 50 wire

cable to 20 MB/s. Yet, it is backward compatible in the same way

that SCSI-2 and Fast SCSI-2 was. Just keep in mind, that if you

put an Ultra-SCSI drive on a plain SCSI-2 controller, your max performance

will still be 5 MB/s. And any old original SCSI devices on a fancy new

ultra-SCSI chain are still stuck at 3 MB/s. But an Ultra-SCSI

controller with Ultra-SCSI drives can get you 20 MB/s.

And following in tradition, there is wide ultra SCSI which does

twice as many bits at the same speed as ultra-SCSI, getting

a maximum theoretical performance of 40 MB/s.

So to summarize. There are devices built for 50 wire buses (50 pin

cables) which can all be connected to the same bus without adapters:

original SCSI 3 MB/s

SCSI-2 5 MB/s

Fast SCSI-2 10 MB/s

Ultra-SCSI 20 MB/s

In general only two devices on a bus talk to each other at any given

time. This is almost always (is always in any case the normal Mac

user will encounter) the controller (logic board or SCSI card) talking

with one of the devices on the chain. These two devices will com-

municate at the rate of the slower device.

And there are devices that are built for 68 wire buses:

Wide SCSI-2 10 MB/s (never seen one of these, don't think they

were ever built)

Fast & Wide SCSI-2 20 MB/s

Wide Ultra SCSI 40 MB/s

Wide devices can be adapted to narrow busses and narrow devices can be

adapted to wide busses, but when you do those devices will only give you

narow performance. And putting narrow devices on a wide bus may slow

down the performance of that bus with the wide devices.

Finally there are some 80 pin devices out there, but I'm not sure what

the deal is with them. I think they're built for hot pluggable

systems. And there are what are called Differential drives. These

use a differnet method of interpreting electrical signals as data,

so that they can better tolerate environments with a lot of

electrical noise and use longer cable runs. Differnetial drives

are not compatible with other SCSI drives and can cause damage if

attached to a regualr bus. There are some adapters for differential

drives, but they are so expensive, you are better off just getting

the correct drive in the first place.

Hope this helps. It's not concis...oh well.

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