27 Nov
2007
27 Nov
'07
10:22 p.m.
Josh Dersch wrote:
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?'
I believe some Microsoft Bus Mouse cards had this facility too. I know I
modified a 24 line parallel card (8255 based, the Maplin kit if anyone
remembers that) to work there (I didn't get a real IBM Async card in my
first XT, I bought a clone 2-port one that wnet in one of the other
slots, but I wasn't goign to waste slot 8...)
One particularly clueless piece of mechanical design is that the chassis
of a 5155 PortablePC is about 1/4" to short to fit the Async card in
slot 8. Thus slot 8 is essentially a wasted slot in that machine.
-tony
However, my 5271 originally (a long time ago,
before I inherited it)
had a card with a BNC connector on the back, which sounds like it may
have been a host interface.
A long time ago we used some PC/3270s and when they
were junked I saved
a few of the cards with BNC connectors so that I could salvage the BNC
connector. I have one left. It is 8-bit and almost square. There is some
standard TTL, one Toshiba package (gate array ?), one IBM SLT package,
and a pulse transformer. It has a DIP switch with a single switch to
select slot 8 or slots 1-7.
What was special about slot 8?