The current loop is actually "proper" .
There are 3 parts to current loop
1) transmitter (switch)
2) receiver (opto coupler in dec stuff)
3) current source
You will have problems if things don't match. An active transmitter has to
connect to a passive receiver.
Passive transmitter to active receiver.
Active means it has a current source,passive no current source.
The 11/05 is active tx active rx. So expects contacts on the keyboard and
the selector magnet in the printer.
Most industrial current loop converters are not this way out of the box.
Now, if this is a VT50, according to my printset it should be passive and
thus if polarity matches should work (unless broken of course). Looking in
the VT52 printset the current loop adapter seems to be configureable using
jumpers for either active or passive. The same goes for the VT1XX option on
the VT100 which had two switches which one could set.
On Dec 1, 2015, at 1:56 PM, Paul Koning
<paulkoning at comcast.net> wrote:
> On Dec 1, 2015, at 1:22 PM, william degnan <billdegnan at gmail.com>
> 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
> 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 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
> 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
> is more like a low impedance voltage sensor,
while the transmitter
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
Hm. I wonder by what definition it is not "proper". A current loop
transmitter, in an electronic terminal as opposed to a mechanical one,
would be a switching transistor that turns the current on and off. The
only issues I can think of are the voltage rating used, and the polarity.
For example, in earlier 60 mA loops (for the Model 15 Teletype and machines
of that era) you might find loop voltages around 100 volts or so, with a
big series resistor. The purpose was to reduce the distortion from the
inductance in the loop (in the receive solenoid). In the later 20 mA loops
I would expect lower voltages to be used. But the specification is pretty
wide open -- "20 mA" is really about all there is. You can build
conforming devices with optocouplers, switching transistors, solenoids and
cam operated switches, etc.
I don't know what "simply switches voltage at the levels +3.5V to -15 V"
means. The voltages at the pins aren't directly relevant, only the voltage
difference between the two transmit pins -- or more precisely, the
resulting current in the loop. I could easily imagine that in the off
state both pins are at -15 while in the on state one goes to +3.5, or
something like that, driving through a resistor of a bit under 1000 ohms.
The fact that the "industrial converter" didn't work does not imply that
the VT50 would have troubles. Just the opposite, actually; I would assume
that DEC would make sure that its 20 mA terminals work with its 20 mA host
Did you check the voltage and current in the two loops when you hooked
it up? I
wonder if you might have the polarity backwards on one of the
circuits, assuming that the devices involved care.