27 Nov
2007
27 Nov
'07
11:49 p.m.
------------Original Messages:
Date: Tue, 27 Nov 2007 22:22:09 +0000 (GMT)
From: ard at p850ug1.demon.co.uk (Tony Duell)
Subject: Re: IBM 5271 (PC3270) host interface options
<snip>
What was special about slot 8?
In a true-blue IBM PC/XT (5160) or PortablePC (5155), and AFAIK very few, if
any clones, the data bus on slot 8 is wired to the opposite side of a bus
buffer to the data bus to the other 7 sltos. For that reason, the card in
slot 8 has to assert (pull low) one pin on the conenctor on read cycles
to that board (I forget the exact pin, I can look it up if you need it).
The IBM Async card (RS232 card) was one card that could do this (jumper
selected). I have heard that one reason such a card was included with
every PC/XT was that it was bout the only thing that could go in slot 8,
and by putting it there at the factory it avoided too many calls from
customers 'Why doesn't the XYZ card work properly in my new XT?'
<snip>
-tony
------------Reply:
Wasn't it used (and intended) for the expansion chassis card?
m
28 Nov
28 Nov
11:28 p.m.
[Slot 8 in a PC/XT]
Wasn't it used (and intended) for the expansion
chassis card?
Not as far as I can see. The XT 'Guide to Operations' states that the
only card that can go in slot 8 is the Async adapter (and it does mention
the epxaniso unit elsewhere, so it's obvious IBM were considering that
option in that manual).
Looking at the scehamtics for the Expanison Unit it the O&A TechRef, it
appears that pin B08 (the pin in question that has to be asserted on read
cycles) is passed on from the expansion unit to the main system, but that
the expanison unit itself never asserts it. So I suppose you _could_ put
the expansion interface card in slot 8, but then (a) the exp[ansion unit
self-tests oulw fail, and (b) the only thing you coulf put in the
expanison unit would be Async cards.
-tony
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