Finally found another old message. This one is from the early days of the LEM Lists before they went to Google Groups.
=======================================
SCSI Voodoo exists because SCSI works when it shouldn't, not because
it doesn't work when it should. So folks misconfigure their SCSI
chains, the machine works anyway, they think everything is fine, then
one day it stops working and they declare SCSI Voodoo.
If your cables are short and your SCSI bus is slow it will still work
without termination, sometimes. The things which termination deal
with are into the realm of analog electronics as opposed to digital
and so it's not an all or nothing deal. Sometimes poor or no
termination is still good enough.
A great site: http://www.scsifaq.org .
Basic SCSI:
1) I will use the terms chain and bus pretty much interchangably.
2) Every device on the SCSI bus must have a unique SCSI ID. It
doesn't matter what order the IDs are in, just as long as they are
each unique.
3) A "device" is anything on the bus, including the host computer, a
SCSI card, disk drives, scanners, etc.
4) No branches are allowed in a SCSI chain, i.e. no Y formations.
5) Every SCSI bus/chain has two ends and only two ends. It's a line
because there are no Ys.
6) Each end of the SCSI bus must be terminated.
7) You, the user, may only need to provide termination at one end,
because some device (host computer, or SCSI card) may be providing
termination automatically for the other end of the chain.
8) Here are some example SCSI chains:
Key
===== internal SCSI ribbon cable
:T or T: A SCSI terminator or termination on a SCSI Device
Dn A disk device of some type, such as a disk drive, n is an integer
which I'm just using as a label for easy communication. It has
nothing to do with SCSI IDs.
C A connector on a SCSI cable with no device attached
MB The host motherboard, or a SCSI card
-------- external SCSI cable
Example 1, Internal SCSI only; Good Termination
T:MB===D1====C====D2:T
The motherboard provides termination at one end and the device at the
end of the chain provides termination at the other end. Notice that
the device providing termination is on the last connector on the
cable.
------------------------------------------------------------------------------------------------------------
Example 2, Internal SCSI only; Bad Termination
T:MB===D1====D2:T====C
The motherboard is still good, but now the terminating device at the
other end of the chain is no longer at the end of the cable. Unless
the distance from D:T to C is only a few inches this is a SCSI no-no.
------------------------------------------------------------------------------------------------------------
Example 3, Internal SCSI only; Bad Termination
T:MB===D1:T====C====D2
The motherboard is still good, but the second terminating device is
not at the end of the cable where it belongs. D1 should have
termination disabled and D2 should have it enabled, or their
positions on the cable could be reversed.
------------------------------------------------------------------------------------------------------------
Example 3a, Internal SCSI only; Bad Termination
T:MB===D1====C====D2
The motherboard is still good, but there is simply no termination for
the other end of the chain. This is bad as both ends of the bus
must be terminated.
------------------------------------------------------------------------------------------------------------
Example 4, External SCSI only; Good Termination
T
2----D1------D3------MB:T
Note that this looks pretty much exactly like good internal
termination. The biggest difference to note is that with external
devices one usually leaves the on-board (built-in) termination turned
off and uses an external terminator module. The module plugs into
the unused cable connector at the end of the SCSI chain. Every
external SCSI device should have two SCSI connectors. One for each
of the two external cables that connects it to the neighboring
devices. When you reach the end of the chain, there's only the one
incoming cable, so the other cable connector is available for a
terminator module to plug into.
Also note that if you use an external module, it is very difficult to
incorrectly terminate your external SCSI chain. You can only plug
the terminator into the last device on the chain, becuase it's the
only one with an empty cable connector. However, you can still
screw up by enabling termination on one of the intermediate devices
on the chain.
Some external devices have only one SCSI connector and have
termination permanently enabled. Such devices must go at the end of
the SCSI cable.
------------------------------------------------------------------------------------------------------------
Example 5, External SCSI only; Bad Termination
T2----D1:T------D3------MB:T
In this case SCSI device D1 somehow has termination enabled. It is
not at the end of the SCSI chain so this is wrong. There are two
primary ways this could happen. D1 could be a disk drive of some
sort and the user left the "Termination Enable" jumper installed so
that D1's built-in termination is still enabled. Or the case in
which D1 is installed may have a built-in termination feature and it
may be switched on. Some of the APS brand cases came with this
termination feature.
So, when you put SCSI devices on an external SCSI chain be sure to
disable the built-in termination (remove the SCSI Term Enable
jumper), unless you will not be using an external terminator, and the
device will be at the end of the chain. But using the built-in
termination is a bad practice because you cannot see at a glance
whether termination is enabled on a device. And you would have to
pull the device out of its case to check the jumpers.
------------------------------------------------------------------------------------------------------------
Example 6, External SCSI only; Bad Termination
T2:T----D1------D3------MB:T
This example builds on the information in example 5. The last
device on the chain is terminated but it is double terminated. This
happens when the user leaves the "Termination Enable" jumper
installed on the SCSI device and installs an external termination
module on the unused cable connector on the last device. This is
also a big no-no.
------------------------------------------------------------------------------------------------------------
Example 7, External SCSI only; Bad Termination
D2----D1------D3------MB:T
This one is trivial. There is no termination at the left end of the
SCSI chain. Termination needs to be provided in some manner.
------------------------------------------------------------------------------------------------------------
Example 8, Internal & External SCSI; Good Termination
T2----D1------D3------MB===D5====C====D4:T
Most Macs which shipped with built-in SCSI have both an internal and
an external SCSI connector and these connectors are both on the same
SCSI bus. In a PCI Mac such as the x500 and x600 series which have
two SCSI busses, this internal/external SCSI bus shows up as Bus 1 in
utilities such as SCSIProbe.
So, in this case we have devices on both the internal connector and
the external connector. This involves two different types of
cabling: a ribbon cable on teh inside and external cable in a chain
of cables and boxes on the outside. However, it is still a straight
line. The rules still apply. There must be a terminator at each
end of the chain.
So the last device on the ribbon cable must be at the end of the
ribbon cable and it must have termination enabled (usually the
"Termination Enable" jumper installed). The last device on the
external chain must have a terminator installed.
The motherboard detects that it is no longer at the end of the SCSI
chain and automatically disables its built-in termination. This is
documented in the Apple Developer Notes for these machines.
------------------------------------------------------------------------------------------------------------
I'm going to skip the "Bad Termination" examples for internal &
external because they can be drawn from teh examples for internal
only and external only.
Mac models such as the x500 and x600 machines and the 8100 have two
built-in SCSI busses. Each bus must be properly terminated. So an
example would be:
SCSI Bus 0 T:MB===D====C====D:T
SCSI Bus 1 T----D------D------MB===D====C====D:T
The machines with two built-in SCSI busses have two internal SCSI
connectors. One of those connectors is the sole connector for SCSI
Bus 0. The other connector is the internal connector for SCSI Bus 1
and is on the same SCSI bus as the external SCSI connector.
I think that's enough. There's a bunch I could write on SCSI IDs
and SCSI cards, but there's a length limit on messages to the list.
Jeff Walther
=========================================================
Still looking for one of my old messages, with similar examples and diagrams about mixing narrow and wide devices...
=======================================
SCSI Voodoo exists because SCSI works when it shouldn't, not because
it doesn't work when it should. So folks misconfigure their SCSI
chains, the machine works anyway, they think everything is fine, then
one day it stops working and they declare SCSI Voodoo.
If your cables are short and your SCSI bus is slow it will still work
without termination, sometimes. The things which termination deal
with are into the realm of analog electronics as opposed to digital
and so it's not an all or nothing deal. Sometimes poor or no
termination is still good enough.
A great site: http://www.scsifaq.org .
Basic SCSI:
1) I will use the terms chain and bus pretty much interchangably.
2) Every device on the SCSI bus must have a unique SCSI ID. It
doesn't matter what order the IDs are in, just as long as they are
each unique.
3) A "device" is anything on the bus, including the host computer, a
SCSI card, disk drives, scanners, etc.
4) No branches are allowed in a SCSI chain, i.e. no Y formations.
5) Every SCSI bus/chain has two ends and only two ends. It's a line
because there are no Ys.
6) Each end of the SCSI bus must be terminated.
7) You, the user, may only need to provide termination at one end,
because some device (host computer, or SCSI card) may be providing
termination automatically for the other end of the chain.
8) Here are some example SCSI chains:
Key
===== internal SCSI ribbon cable
:T or T: A SCSI terminator or termination on a SCSI Device
Dn A disk device of some type, such as a disk drive, n is an integer
which I'm just using as a label for easy communication. It has
nothing to do with SCSI IDs.
C A connector on a SCSI cable with no device attached
MB The host motherboard, or a SCSI card
-------- external SCSI cable
Example 1, Internal SCSI only; Good Termination
T:MB===D1====C====D2:T
The motherboard provides termination at one end and the device at the
end of the chain provides termination at the other end. Notice that
the device providing termination is on the last connector on the
cable.
------------------------------------------------------------------------------------------------------------
Example 2, Internal SCSI only; Bad Termination
T:MB===D1====D2:T====C
The motherboard is still good, but now the terminating device at the
other end of the chain is no longer at the end of the cable. Unless
the distance from D:T to C is only a few inches this is a SCSI no-no.
------------------------------------------------------------------------------------------------------------
Example 3, Internal SCSI only; Bad Termination
T:MB===D1:T====C====D2
The motherboard is still good, but the second terminating device is
not at the end of the cable where it belongs. D1 should have
termination disabled and D2 should have it enabled, or their
positions on the cable could be reversed.
------------------------------------------------------------------------------------------------------------
Example 3a, Internal SCSI only; Bad Termination
T:MB===D1====C====D2
The motherboard is still good, but there is simply no termination for
the other end of the chain. This is bad as both ends of the bus
must be terminated.
------------------------------------------------------------------------------------------------------------
Example 4, External SCSI only; Good Termination
T
2----D1------D3------MB:TNote that this looks pretty much exactly like good internal
termination. The biggest difference to note is that with external
devices one usually leaves the on-board (built-in) termination turned
off and uses an external terminator module. The module plugs into
the unused cable connector at the end of the SCSI chain. Every
external SCSI device should have two SCSI connectors. One for each
of the two external cables that connects it to the neighboring
devices. When you reach the end of the chain, there's only the one
incoming cable, so the other cable connector is available for a
terminator module to plug into.
Also note that if you use an external module, it is very difficult to
incorrectly terminate your external SCSI chain. You can only plug
the terminator into the last device on the chain, becuase it's the
only one with an empty cable connector. However, you can still
screw up by enabling termination on one of the intermediate devices
on the chain.
Some external devices have only one SCSI connector and have
termination permanently enabled. Such devices must go at the end of
the SCSI cable.
------------------------------------------------------------------------------------------------------------
Example 5, External SCSI only; Bad Termination
T
2----D1:T------D3------MB:TIn this case SCSI device D1 somehow has termination enabled. It is
not at the end of the SCSI chain so this is wrong. There are two
primary ways this could happen. D1 could be a disk drive of some
sort and the user left the "Termination Enable" jumper installed so
that D1's built-in termination is still enabled. Or the case in
which D1 is installed may have a built-in termination feature and it
may be switched on. Some of the APS brand cases came with this
termination feature.
So, when you put SCSI devices on an external SCSI chain be sure to
disable the built-in termination (remove the SCSI Term Enable
jumper), unless you will not be using an external terminator, and the
device will be at the end of the chain. But using the built-in
termination is a bad practice because you cannot see at a glance
whether termination is enabled on a device. And you would have to
pull the device out of its case to check the jumpers.
------------------------------------------------------------------------------------------------------------
Example 6, External SCSI only; Bad Termination
T
2:T----D1------D3------MB:TThis example builds on the information in example 5. The last
device on the chain is terminated but it is double terminated. This
happens when the user leaves the "Termination Enable" jumper
installed on the SCSI device and installs an external termination
module on the unused cable connector on the last device. This is
also a big no-no.
------------------------------------------------------------------------------------------------------------
Example 7, External SCSI only; Bad Termination
D2----D1------D3------MB:T
This one is trivial. There is no termination at the left end of the
SCSI chain. Termination needs to be provided in some manner.
------------------------------------------------------------------------------------------------------------
Example 8, Internal & External SCSI; Good Termination
T
2----D1------D3------MB===D5====C====D4:TMost Macs which shipped with built-in SCSI have both an internal and
an external SCSI connector and these connectors are both on the same
SCSI bus. In a PCI Mac such as the x500 and x600 series which have
two SCSI busses, this internal/external SCSI bus shows up as Bus 1 in
utilities such as SCSIProbe.
So, in this case we have devices on both the internal connector and
the external connector. This involves two different types of
cabling: a ribbon cable on teh inside and external cable in a chain
of cables and boxes on the outside. However, it is still a straight
line. The rules still apply. There must be a terminator at each
end of the chain.
So the last device on the ribbon cable must be at the end of the
ribbon cable and it must have termination enabled (usually the
"Termination Enable" jumper installed). The last device on the
external chain must have a terminator installed.
The motherboard detects that it is no longer at the end of the SCSI
chain and automatically disables its built-in termination. This is
documented in the Apple Developer Notes for these machines.
------------------------------------------------------------------------------------------------------------
I'm going to skip the "Bad Termination" examples for internal &
external because they can be drawn from teh examples for internal
only and external only.
Mac models such as the x500 and x600 machines and the 8100 have two
built-in SCSI busses. Each bus must be properly terminated. So an
example would be:
SCSI Bus 0 T:MB===D====C====D:T
SCSI Bus 1 T
----D------D------MB===D====C====D:TThe machines with two built-in SCSI busses have two internal SCSI
connectors. One of those connectors is the sole connector for SCSI
Bus 0. The other connector is the internal connector for SCSI Bus 1
and is on the same SCSI bus as the external SCSI connector.
I think that's enough. There's a bunch I could write on SCSI IDs
and SCSI cards, but there's a length limit on messages to the list.
Jeff Walther
=========================================================
Still looking for one of my old messages, with similar examples and diagrams about mixing narrow and wide devices...