[GreenKeys] Seems like old times...

[Jim Haynes]

Meanwhile, not being really ready to take on HeavyMetal, I got a
different setup running.  Back circia 1970 I designed a speed converter
taking input at 45.45, 50, xx, and 74.2 baud  where xx is the rate for
75 wpm, which I don't recall at the moment.  It puts out 74.2 baud.
So it's an electronic gear shift (receive only).  This was back before
we even had UARTs, so it's all done with TTL.  (For anyone who has a
morbid interest in such things a slightly better design is written
up in Ham Radio magazine, December 1971, p. 36.)

So I've just been making a few experiment boxes: a power supply to run
the thing, and one holding my circa-1970 circuit that converts between
a modified RS-232 signal level and 60ma loop, and one that opto-isolates
converting 60ma loop to modified-RS-232.  And hooked it all together and
it started working, with the M28 geared for 100 wpm.  It sounds a little
strange running at 74.2 baud, 60 wpm.  (And even stranger at 66 wpm.)

The modified RS-232 (I was afraid you'd never ask) basically has a
diode in series with the transmitter output, forming part of a
positive OR gate such that several senders and receivers can all be
put in parallel on a hub and any sender that goes spacing (positive)
will drive the hub spacing.