11 Feb
2006
11 Feb
'06
3:22 p.m.
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