11 Feb
2006
11 Feb
'06
4:07 p.m.
On Saturday 11 February 2006 18:22, you wrote:
The writeup that IBM published at the time, said they handled slot 8
differently to support the expansion interface. The bus driver was normally
off unless a device the the expansion chassis was addressed.
I assume it would be for RFI reduction, but that's a guess on my part.
I've never seen docs on the expansion interface, so can't verify it.
joe lang
Under
"puzzles for the ages", one might also wonder what the IBM was
thinking when they added the 20ma current loop circuitry to the board.
Did they really think that folks were going to use them to drive an ASR
33? Or that RS232-to-current loop adapters were hard to obtain?
A few random guesses (as I haev said before, you can find out _what_ was
done (e.g. by looking at schemaitcs or listings), but not in general _why_)
1) The IBM PC had similar design ideas to the Apple ][ (open machine,
basic/cassette port, etc). One of the early Apple ][ serial boards had a
current loop intenrface (and the manual for that _did_ describe how to
connect it to an ASR33)
2) IIRC IBM made some current loop printers. A friend of mine has a
System 23 (Datamaster), I seem to remember the printer on that has a
current loop interface
3) There were other current loop devices around, maybe the extra
circuitry was added for people who wanted to use thoase Why have to
bother with an ecternal converter unit?
-tony