What does the terminator do?

[David Yandry]

I have been experiencing intermittant problems with several devices on the same SCSI bus. There has been no pattern that I am able to identify so the repairman suggested that I replace the terminators. I've had no more problems for two days so maybe this fixed but we sometimes went for several days without a glitch. My repairman couldn't tell me exactly what the terminator does but he said that he had seen this kind of problem before. What does the terminator do?

Thanks,
David

[Jeff Schussele]

Hi David,

It essentially cancels out the SCSI signals at the ends of the bus so that they don't "bounce" around the bus so to speak. If they don't get terminated they can cause trouble by throwing off the timing when they re-arrive back at a device unexpectedly.

Rgds,
Jeff

[A. Clay Stephenson · Accepted solution]

Think of a rubber ball striking a hard wall. -- this is your SCSI signal without a terminator. Now think of a rubber ball striking a wall of dough -- this is your SCSI signal with a terminator.

I'm actually serious about this answer; it's quite surprising how many electrical network configurations have direct mechanical equivalents and often the equations which describe them are exactly the same.

[Patrick Wallek]

Besides run for governor of California? Sorry, I couldn't resist.

Have a look at this site:
http://www.pcguide.com/ref/hdd/if/scsi/cablesTermination-c.html

Hopefully it will explain scsi termination.

[A. Clay Stephenson]

I should add the the kinds of problem you are seeing can also be caused by SCSI bus lengths that exceed (or even closely approach) the published maximum lengths -- especially if this is SE-SCSI. A faulty controller can cause those kinds of well. You should also make certain that at least one device on the bus is supplying termination power --- not the same as termination.

By the way, terminators on the older 10-Base-2 and 10-Base-5 network cables serve exactly the same purpose.

[Chris Vail]

Fundamentally, it keeps SCSI signals in sync. The faster the SCSI bus, the more important this becomes.

The problems you mention could indeed come from a bad terminator. They could also be from too much cable in the SCSI chain. There may be too many connectors in the SCSI chain (this one is particularly frustrating to isolate). Lastly, there may be too many devices in the SCSI chain. Keep the cables as short as possible and the connectors as few as possible. And have spare cables handy to isolate such problems.

AFAIC: no more than 2 devices plus the controller per SCSI chain. I know that the specification is for 7 or 14, but the more devices on the chain, the more likely for these kind of random errors to occur. You can put more of the slower tape devices on a SCSI chain, but my hard-and-fast rule is that no more than 2 high speed disk or tape devices on a chain. In theory, you can put more than this, but that IS theory. My practical experience is to use LOTS of controllers when LOTS of devices are necessary.

Be sure you always have the latest version of the microcode on both the SCSI host adapter and all SCSI devices. You'd be amazed at how fixing the microcode level can end these random errors.

Lastly, consider moving out of copper SCSI over to fiber channel. Yes, its a lot more expensive. But fiber is considerably more reliable than copper, and these random errors (for which SCSI is famous) are almost eliminated. Distance limitations are almost a thing of the past, and you can do cool stuff like building a SAN with fiber that is really, really difficult with copper.


Chris

[Gerhard Roets]

Hi David

A terminator makes a clean cabling end to set up correct wave forms in the cable. This also prevents extra noise on the cable but thats a small feature that can be negated by large degrees of noise.

If you have bad termination this implies that you will get bad SCSI signals and hence problems and glitches.

This is just a short writeup there is much more detailed writeups above.

Regards
Gerhard