> One time in college I was building a device to interface with the I/O bus=
 of
> a minicomputer (sortof like a PIC, but slower, larger, more expensive, an=
d
> required its own room with dedicated power transformer and air conditione=
r).
> One of the signalling lines was open collector to be pulled down by the
> device as part of some handshake.

Was it PDP-12 ? What was the power transformer for?
--=20
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