Interfacing PDP 11/05 to VT 50

william degnan billdegnan at
Tue Dec 1 12:22:12 CST 2015

On Tue, Dec 1, 2015 at 12:43 PM, Ethan Dicks <ethan.dicks at> wrote:

> On Tue, Dec 1, 2015 at 11:55 AM, Paul Koning <paulkoning at>
> wrote:
> > I'm not sure I understand the question correctly.  That article clearly
> points out the 20 mA wires, and presumably that's where your ASR33 is
> connected.  The VT50 comes (according to the peripherals handbook) with a
> standard 20 mA interface, optional RS232 interface.  So it sounds like it
> would be a matter of finding where the 20 mA connector on the VT50 is, and
> plugging into that.
> >
> > Interestingly enough, the VT52 is listed as supporting either kind of
> interface, but only one; "specify at time of order".
> I don't know that I've taken apart a VT50 or if there are internal
> differences, but for the VT52, there's a ~2"x5" (from memory) PCB with
> interface circuitry for either EIA or 20mA and a permanently-attached
> cable with the appropriate connector on the host end.  You take the
> bottom of the terminal off, pop the board, pop the other one on and
> it's switched.
> It would not be difficult to replicate either board, either by hand or
> with a fabricated PCB.
> -ethan

Sorry about the wording of my question.  Thanks for the replies.  I was
only able to get the VT50 to receive, I could not send.  So I decided to
research the problem.  I found the link above, the author of the page says
in effect 20mA did not work (for him) as desired.  So I was wondering if
anyone here has been successful to attach a M9970 to a VT50 or VT52.  I
spend more time on it, but I was curious if it was even worth it given the

The author writes:

-start quote--

I first tried to connect a PC with the 11/05 over a industrial
RS232-to-20mA converter, but this failed.

A 20mA interface works by one side providing a 20mA current, which drives
receiver and transmitter of the closed loops for Transmit and Receive.

But the 20mA interface of the PDP-11/05 is not a proper one: the receiver
is more like a low impedance voltage sensor, while the transmitter simply
switches voltage at the levels +3.5V to -15 V.  At best you call the
PDP-11/05 serial interface a "TTY interface": it is well suited to read
data generated by mechanical switches to GND, and driving solenoids for
transmit data.
-end quote--

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