[GreenKeys] 20mA circuit

[B M]

I am almost ready to get my ASR33 hooked up to my PDP-11 computer.

I am looking for a circuit that uses optoisolators to ensure both
sides of the connection are protected from each other.  I do want a
full duplex connection.  I will be using an active 20mA port on the
PDP-11 and a passive 20mA connection on the TTY.  I would prefer to
use as low a voltage that will work on the TTY.  I suspect the PDP-11
uses +/-12v, but I have not yet confirmed that detail.

I only have a basic knowledge of electronics and can build the circuit
if I have a schematic drawing.  I would also be looking for tips on
how I might test the circuit before I actually use it.

Any ideas?

Many thanks.

--barrym