SCSI bus topology questions

[Jim Thompson]

>>>>> "Jeremy" == Smith, Jeremy <SmithJ at mps.bellhowell.com> writes:

    Jeremy> If I'm not mistaken the disable/enable option is for the
    Jeremy> low-byte SE terminator on the SCSI bus. This is the only
    Jeremy> term shared between any of the "plugs". High-Byte UW should
    Jeremy> always be termed in this setup and High/Low-byte U2W should
    Jeremy> always be termed. If both of the SE connectors are in use
    Jeremy> then the Low byte on the bus would not need to be terminated
    Jeremy> by the host adapter as it is in the middle of the bus. The
    Jeremy> Low byte should be terminated with good quality terminators
    Jeremy> on either end of the SE bus. Often times the terms on
    Jeremy> CD-ROMS, tape drive and the like are not of very good
    Jeremy> quality. Try disabling the terminator in the SCSI Bios and
    Jeremy> if that fails get a couple of active terminators to put on
    Jeremy> the ends of your cables.

Let me second Beau's original plea -- I find the P2B-S termination to be
terribly confusing.  Perhaps the way to understand it is to turn the
question around: if I have devices on each of the three bus segments
(LDV, wide-SE, narrow-SE), then: (a) does the last device on each bus
segment need to be terminated? (b) what termination, if any, needs to be
enabled on the motherboard?

Presumably, the answer to (a) would be "yes".  Each cable has a
*physical* end, which needs to be terminated to keep signals from
bouncing from it.

The answer to (b) is what's confusing, and seems to rely on some
assumptions about what's electrically connected to what at the
motherboard (SCSI controller).  If the narrow and wide SE segments are
electrically tied together, then the answer would seem to be "high on, 
low off".

To add to the confusion, both the Award and Adaptec BIOSes have
termination settings.  The Award BIOS two on/off termination settings,
one for high, one for low.  The Adaptec BIOS has only one one/off
setting, simply labeled "termination".

JT
--
     _                                 |                             .~.
   _| ~-    Jim Thompson               |       Yo quiero Linux       /V\
   \,  _}   jim.thompson at pobox.com     |                            /( )\
     \(     http://pobox.com/~kzinti   |                             ^-^

To Unsubscribe: send mail to majordomo at FreeBSD.org
with "unsubscribe aic7xxx" in the body of the message