Need a bus driver solution

Well, it ain't exactly a bus driver, but anyway.

I have 20 TTL signals that I want to transmit over thirty feet of
cable. Max frequency on any of them is 500kHz. Timing isn't that
crucial because these are step/direction signals for a bunch of
stepper motor drives, so what's essentially important is only
that the number of pulses that make it to the other end stays the
same.

Ideas:

1) Use +/- 5V bipolar, single ended RS232-style technology. Found
that the ancient 1488-type chips don't seem to support that
high a data rate, although they're spec'd at 30V/us slew
rate.

2) 20 parallel symmetric twisted pairs.

3) Some form of serialization.

1) and 2) are equally simple and straightforward, except that I'd
like to make do with at most a 25-pin DSUB connector which rules
out 2). As for 1), with 20 single-ended signals running in
parallel over a cheap cable I think crosstalk may become
unmanageable. Or the thing would have to become so low-impedance
that power consumption becomes an issue.

3) I've thought about sampling the inputs at a 1MHz rate,
stuffing them into three 8-bit shift registers and clocking the
data out at 8 MHz over symmetric twisted pairs. That would make
five pairs (three for the data, one clock and one strobe).

But before I begin working on this I'd like to know if there
exist industry-standard, integrated solution for this rather
generic problem.

Solutions that attack the motor connection at another point
(i.e., not at the step/direction level) are out because this is
ancient equipment which works very well. At the moment the motor
drives are next to the control computer (i386) with a bunch of
thick cables running to the motors, and I'd like to put the
drives next to the motors and control the thing over just one
thin cable.

Thanks,

robert
.

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