Interfacing PDP 11/05 to VT 50

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.

Joe wrote: says the drives receiver simply 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. 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 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 interfaces. it up? I wonder if you might have the polarity backwards on one of the circuits, assuming that the devices involved care.