[GreenKeys] 20mA circuit

[Nico de Jong]

>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.
>

I can recommend http://www.baudot.net/ I'm right now installing a 
teleprinter net in our data museum (Denmark). The tleprinter connects to a 
standard COM port. The part between the RS232 chip and the exit to the PC 
port mas been modified a bit to allow for Connect, Disconnect, and "Local" 
buttons. I wrote the software in Delphi 6. The TTY will need more then 
+12VDC, I use a 110-110 isolation transformer. This keeps the contacts 
clean. Everything is built into a box previously used for a SCSI drive.
Most of the hardware is available through Mouser, but the eurocards are 
bought locally. Prints are not available anymore.

I'd be happy to lend a (remote) hand

All the best