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From: urlichs@smurf.sub.org (Matthias Urlichs)
Newsgroups: comp.os.386bsd.bugs,comp.periphs.scsi,comp.os.linux
Subject: Re: 1542C not taking commands!
Followup-To: comp.periphs.scsi
Date: 11 Jun 1993 13:00:48 +0200
Organization: University of Karlsruhe, FRG
Lines: 61
Message-ID: <1v9ol0$gjh@smurf.sub.org>
References: <1993Jun9.125931.537@lssec.bt.co.uk> <hastyC8F3Aw.1C8@netcom.com> <1v8sktINN152@hrd769.brooks.af.mil>
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In comp.os.386bsd.bugs, article <1v8sktINN152@hrd769.brooks.af.mil>,
root@hrd769.brooks.af.mil (Super User) writes:
[ Adaptec 1542C woes ]
>
> WHen someone comes up with a more or less definitive explanation and fix,
> could you drop me a line and help me put this in the FAQ. I really do not
> understand how this fix works, or precisely what the fix is, but it is coming
> up with alarming frequency...
>
The problem is that the Adaptec 1542C has (a) rather powerful line drivers,
and (b) is sensitive to transient signals which can be induced by them via
either a bad cable or a bad external terminator.
A bad cable is almost any cable which doesn't meet SCSI-2 specs.
A bad external terminator is one which doesn't adequately buffer its
resistor network.
So...
- Remove the internal terminator from the last drive in your chain.
Replace with an active SCSI-2 external terminator. Side improvement:
active terminators consume a bit less power.
- Check cables. Specifically, some cables carry less than the nominal 50
signal wires. Manufacturers sometimes think they can get away with this
because almost all odd-numbered pins are GROUND anyway. So, if pins 1
and 3 or 3 and 5 are connected, you're likely to have a marginal cable.
- Make sure that the terminator power is supplied by all devices and that
the power pin is actually connected on your cable. The problem here is
that some idiot device manufacturers save on 2-cent diodes, which means
that the thing will pull terminator power to ground if it's not plugged
in. (Two of these on one bus are even worse.)
- Consider creating your own cabling. Take a 50-wire flat ribbon and press
the appropriate connectors onto it in precisely the right places. (Move
your devices as to minimize cable length.) Be aware that if a device
has two external connetors, you must take the SCSI bus in at one connector
and out at the other -- don't leave the other connector dangling; this
isn't within the SCSI specs because the cable usually is too long.
- Better but more expensive: use 2-twisted cable. (I.e., wire 1&2 are
twisted around each other, wire 3&4, ...) This will improve reliability
because the wires are twisted at different rates. These cables have short
nontwisted segments every 50 cm (1.5') so that you can press on your
connectors instead of heating up that soldering iron.
- While you're rebuilding your system anyway..: If you have more than one
drive per power supply, check if these drives have adequate condensators
to buffer their power. I have two 80-MB Seagates which refused to work
more than a few hours without glitches -- then I soldered two 10-uF Tantals
onto their power connector and they've been flawless ever since.
The terminator power is pin 26. Be aware that SCSI counts pins as they appear
on a ribbon cable, not as they're sometimes numbered on the connectors.
Pin 25 is supposed to be disconnected.
NB: Crossposted (and replies redirected) to comp.periphs.scsi.
Corrections welcome.
--
"If life had a vomit meter, we'd be off the scale."
-- Joe Bob Briggs
--
Matthias Urlichs -- urlichs@smurf.sub.org -- urlichs@smurf.ira.uka.de /(o\
Humboldtstrasse 7 -- 7500 Karlsruhe 1 -- Germany -- +49-721-9612521 \o)/