IBM 5271 (PC3270) host interface options

M H Stein dm561 at
Tue Nov 27 17:49:40 CST 2007

------------Original Messages:
Date: Tue, 27 Nov 2007 22:22:09 +0000 (GMT)
From: ard at (Tony Duell)
Subject: Re: IBM 5271 (PC3270) host interface options

> 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?'


Wasn't it used (and intended) for the expansion chassis card?


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