SUMMARY: SCSI terminators: What's the diff?

From: Daniel Hurtubise (daniel@sar3.CANR.Hydro.Qc.CA)
Date: Fri Mar 10 1995 - 08:22:27 CST

Sun Managers,

My question was on the differences between the types of
SCSI terminators.

>From the answers I received, three types exist.

1) Passive terminators

They attempt to match the electrical voltage on the bus,
but they are just a pass through device. Michel Pearlman

"The original SCSI terminators were passive consisting of a resistor
 based voltage divider and thus if your termination supply voltage varied
 so did your termination."

2) Active terminators

These are the same as regulated terminators. In fact,
active terminators are regulated to maintain voltage regardless of
variations. Furthermore, as Paul Hostrup-Jessen explains:

"Until the SCSI-II standard came (fast SCSI) you were able to use the normal
 passive terminators without any problems. The active terminators (active =
 pull-up resistors which make sure that the pulse transition is as fast as pos-
 sible) are designed for the faster SCSI standards, but can also be used with
 slower SCSI busses. I would stick to the active terminators only, though they
 do cost a bit more."

3) FPT's (Forced perfect terminators)

These are the best and you should accept nothing less than these if
you are using fast-wide or fast-wide differential devices and controllers.

Other interesting points were made regarding cables. Use matched impedance
cables on the same chain and use the shortest cable length possible. Also,
Sun is now shipping scsi cables that have collars (chockers) on each end.
These are the ferrite type and some say they solve a lot of the problems
with cabling.

Apparently, there was a talk on this (which I seemed to have missed) at the
last USENIX conference. You can check the ftp sites or for more information. Also, the news group comp.periphs.scsi
has a FAQ for SCSI devices.

Thank you to the respondants for their concise answers.

Daniel Hurtubise
Systeme d'Analyse de Reseau
C.A.N.R., Hydro-Quebec

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