> Why would anybody do such a thing?   Was this just an experiment in
> twisted design or was there a good reason?

Coax was used for many reasons: If you are driving a m/bit per second
fast rise digital signal over distance and you want to preserve the
waveshape (A 5 Mhz sq wave may have components well into the 100's of
Mhz), you need to match the characteristic impedance of the line at both
ends to prevent reflections and other waveform degradation - coax is the
easiest way to handle this. Coax also provides screening to meet all the
rfi regs. Finally, it allows you to use a cheap coax connector and
cable, rather than multiway connectors, which (for quality) cost an arm
and a leg, as well as having reliability problems. The extra bit of
electronics to serialise and deserialise the bit stream is peanuts in
comparison with the advantages. Over distances like 30 meters (~100ft)
it's doubtfull if it could have been done any other way. ISTR, the data
rate was in the tens of m/bits/second - thinking about it, would have to
be to get any throughput...

Remember, Dec have always been first and foremost an _ hardware
engineering_ company. Expect to find a soundly engineered solution...

Chris