.Current Loop: Design Notes

Home » .Current Loop: Design Notes

This page has notes of things I’ve discovered in relation to the current-loop interface as used on PDP-8 and PDP-11 computers.

DEC Current loop interface boards

DEC had some interesting small boards that were used to interface between current loop and particular peripherals.

I don’t have many of these, but I do have some interesting documentation about them:

Sample Circuits

  • PDP-11/10
  • DLV11 (M7940) as shown in the 11/03 Schematics. Nice simple circuit
  • DLV11-F (M8028) as shown in Print Set MP00461. Similar to the DLV11 circuit, but with a modified TX driver (extra 3 transistors and 6 resistors).

The DLV11 (M7940) schematics are covered in the DLV11 Field Maintenance Print Set (Print Set Order Number MP00055). Information about the DLV11, such as jumper settings, can be found at pages 211-216 of the Microcomputer Interfaces Handbook 1983-84 (Order Code EB-23144-18).

The DLV11 and DLV11-F have virtually identical circuitry for the reader-run and receive-data loops. The transmit-data loop is also identical when in “passive” mode (ie where the loop is powered by the device at the other end of the cable).

The transmit-data loops differ significantly when in “active” mode. Here are the key differences:

  • DLV11 (M7940): The loop is powered by +12V through two series resistors (270R and 180R). These resistors drop a total of 9.0V at 20mA. This leaves a total of 3.0V for the transmitter, receiver and the cabling between them (including the voltage necessary to overcome cable capacitance). This seems to be a somewhat marginal design.
  • DLV11-F (M8028): The loop is powered by +12V, and switched by a pair of output transistors (PNP from +12V and NPN to GND) in series with a pair of 47R resistors. Each transistor is current limited to 36mA. This circuit will drive a 10.1V load at 20mA, or an 8.6V load at 36mA.


Some DEC designs use an MC1301 diode. That diode provides a constant current of 1mA. Datasheet is here.

The MCL1301 does not seem to be available from usual distributions. However, as at June 2017 Anchor Electronics appears to have them.

An equivalent appears to be the Semitec E-102. Datasheet is here.

Interface Specification

I’ve not been able to find a definitive specification for the current loop interface.

However, the VT100 Technical Manual (EK-VT100-TM-003) has (at Page 3-6) a good description of minimum/maximum values for both mark and space states. Here is an extract from that page.

Section 6.3 of the same VT-100 manual has a good description of the the circuitry works (see Section 6.3, pages 6-18 to 6-22).

There is also useful information in the M8650 manual.

B&B Electronics Current Loop Application Note also has lots of good information about current loop design.