SCSI termination

[Jun Zhang_4]

I have two E35 servers. One of them need a termintor plugged to the mini 50 pin external connector on the back of the box to be able to boot. Only a disk and a tape drive in the box. Don't see anywhere to toggle the termination for the disk, but I made sure that the tape drive has the term power at the ON position.
The other E35 doesn't need the terminator plugged in the back, and it boots fine.
What is the way I can make the first E35 behave like the second one?

Jun Z.

[A. Clay Stephenson]

Welcome to the world of SCSI. The box that is running without the terminator is running by accident. All SCSI buses MUST be terminated in EXACTLY TWO PLACES -- on the ends of the bus. Often the controller has built-in termination that can be enabled.

Strangely, unterminated SCSI buses will almost work perfectly. You got lucky in one case but almost certainly if that bius got very busy, you would start seeing problems.

The SCSI terminator serves much the same purpose as a "wall of dough" does for a ball. Instead of bouncing off, the ball is stopped. Likewise, terminators stop the reflection of electrical signals back down the bus.

[Jun Zhang_4]

Hi Clay,
Can I further ask,
How the ends of the bus are defined?
What happens if more than two terminations exist in one bus?

Jun Z.

[A. Clay Stephenson]

The ends od the bus mean the physical ends of the entire SCSI bus. You put terminators on the last physical device on each end of the bus.

This is how your bus should be configured:

TERM -- Controller ---- Disk ---- Tape --- Disk --- TERM

In the one case, one of the terminators is missing but the bus continues to work (more or less).

If you run, with 0,1,3 or any number of terminators other than 2 OR if the terminators are not installed at the physical ends of the bus, the behavior is undefined.

[Jun Zhang_4]

I didn't mean to go further from this point, since something I don't really understand, so that I may not present it with enough sense, but what I'm really trying is to combine the two scsi bus of the two E35 through using an external disk. Now it looks something like,
Disk--Tape--Control--D--Control--Tape--Disk
<----- E35 1 ------>|<------ E35 2 ----->
The middle D is a disk outside both boxes. Suppose to the left of the middle D is the box need an external terminator to boot. In my situation, once I made such connection, ioscan -fnC disk from the right E35 sees the disk to the far left, for a few seconds, and then the whole thing collapse. Pull the cable from the left of D, E35 2 come back, plug the terminator to the left controller, E35 1 is back.
The left tape is terminated, and there got to have one device or two terminated on the right (otherwise E35 2 couldn't be normally operating for the past year).
The next thing to try may be to have the middle D term power enabled in favor of E35 1? Any suggestion?

Jun Z.

[A. Clay Stephenson]

As long as at least one device on the bus has termination power enabled that is fine. It is okay for more than one device to supply termination power. That is absolutely not the same thing as a terminator.

A terminator looks like this for every data line on the bus:
+V --- resistor ---^------resistor --- GND
The signal lines are connected at the "^". This is how passive terminators are constructed; active terminators use transistor networks to simulate the same circuit.

Your SCSI bus should look like this:
Host A:
TERM -- Disk --- Controller (SCSI ID 7) ---<
Host B + External Disk:
<---- External Disk ---- Controller (SCSI ID 6) --- Disk --- TERM.

The --< and <-- are plug together.

I suspect that your fundamental problem is that either you have not set the SCSI ID of one of the controllers to 6. Every device on the combined bus MUST HAVE A UNIQUE SCSI ID.
The other "gotcha" for SE-SCSI is that it is very easy to exceed the absolute maximum bus length of 6m -- especially when combining two boxes. To even reach 6m requires very high quality cable.

[Jun Zhang_4]

The actual bus I have is like the following,

D--T--C--ED--C--T--D--D
5 1 7 3 6 4 2 0

D-- Disk, T-- Tape, C-- Controller, ED-- External disk.

The total length of the external cable is about 2 meters.

Jun

[Jun Zhang_4]

Does the Term Power serve the purpose to do termination? If not, what is the difference between on and off?
There is only one location to plug a terminator for each E35. My E35 (A) need it, my E35 (B) doesn't need it. Is this has something to do with the auto termination feature associated with the controller?


Jun

[A. Clay Stephenson]

Your diagram MUST be wrong because there are no terminators displayed. This means that the disks on the ends the bus MUST have termination enabled OR there must be a terminator on the end of the internal cable. Internal SCSI cables often have a integral terminator.

If the jumper is marked "TERM PWR" or "TERM POWER" this is NOT termination. It simply means that when "TERM PWR" is on, it is supplying +V as indicated in my terminator diagram above. There is a SCSI line dedicated to this purpose.

Finally, the 6m SE-SCSI bus length limit apllies not just to external cables but the entire bus. All internal and external cabling as well as any that loops within a device itself add to the total.