Date : Mon, 22 Oct 2007 16:37:04 +0100
From : adsb@... (Andrew Benham)
Subject: ACB4000 on modern SCSI adaptor ?
Pete Turnbull wrote:
> Almost certainly neither. Smoke from a SCSI cable is because something
> on it is providing TERMPWR but the cable is upside-down on one
> connector, and therefore TERMPWR is shorted to ground. Pretty much
> everything else on the cable is open-collector and won't be harmed by
> being upside down or wrongly connected.
Right, I think I've got to the reason why I remembered something about
this problem.
SASI didn't have a TermPwr line, SCSI does.
On the 50-pin SCSI connector, single-ended SCSI uses pin 26 for TermPwr;
for differential SCSI pins 25 and 26 are both used for TermPwr.
On single-ended SCSI, all the odd number pins are to be grounded EXCEPT
for pin 25 - this is left open so that if a single-ended device is
plugged onto a differential SCSI bus it doesn't short TermPwr to ground
(or if the connector is put on backwards, it doesn't short TermPwr)
[ Having said this, I'm looking at a circuit diagram from a 1986 Byte
magazine which joins pins 25 and 26 and uses them both for single-ended
TermPwr. ]
Now, my ACB4000 manual from 1983 tells me that "all odd pins are
grounded" on the SCSI bus - i.e. including pin 25.
So I got my old Xebec S1410 controller and my spare ACB4000 out of the
cupboard, and stuck the AVO on them.
The S1410 has ICs with late 1984 dates on them. Pin 25 is open.
The ACB4000 has ICs with late 1983 dates, and pin 25 is grounded.
Neither of the boards have pin 26 grounded.
So I reckon the change in the recommendation for pin 25 came along in
1984.
Now some of the modern day SCSI adaptors are actually LVD but will
switch to single-ended mode if required. But they won't be expecting
pin 25 to be grounded - for over 20 years it's been specified to be
open (for single-ended) or TermPwr (for differential).
This could be the reason for the smoke (another reason would be getting
the connector on backwards, which wouldn't be a problem if pin 25 was
open).
So my plan, which I've seen mentioned elsewhere [1], is to bend pin
25 on the ACB4000 SCSI connector out of the way, or cut it off at its
base, so that connection can never be made to it. On the ACB4000 the
SCSI connector is two rows of Berg pins, and the odd numbered side is by
the edge of the board which makes decommissioning pin 25 quite easy.
1/.
http://apple2.org.za/gswv/a2zine/GS.WorldView/v1999/Jul/BEST.OF.A2.USENET/Sider_Info_MORE.txt
--
Andrew Benham adsb@...
Southgate, London N14, United Kingdom
The gates in my computer are AND OR and NOT, not "Bill"