[GreenKeys] 20mA circuit

[Jim Haynes]

The Model 32/33 machines internally are electrically isolated from ground,
so you don't need optoisolators.  The Model 32/33 machines do not have
inductors in the line circuits, so you do not need high voltage and high
resistance like you would with a 28 or earlier machine.  Why they are
current-loop at all is a mystery that only Bell Labs people knew.

If you have a PDP-11 with a 20ma interface it is already designed to
connect directly to the Model 33.  Depends on the kind of interface board
you have in the PDP-11.  The original KL-11 was current loop only; the 
later DL-11 could handle current loop or RS-232.  The only thing DEC
did was to bring out a relay that has the contacts in series with the
tape reader stepper so they can control when it reads tape.  So you
should have a 6-wire interface from the PDP-11.

Since the 32/33 were designed for low-voltage operation you also don't
need a high voltage across the contacts to keep them clean, like you
do for earlier machines.