1 Dec
2015
1 Dec
'15
10:22 a.m.
On Tue, Dec 1, 2015 at 12:43 PM, Ethan Dicks <ethan.dicks at gmail.com> wrote:
On Tue, Dec 1, 2015 at 11:55 AM, Paul Koning
<paulkoning at comcast.net>
wrote:
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
hardware.
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--
--
Bill
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