VT220 current loop interface

Home » Current loop » VT220 current loop interface
Current loop No Comments

This post takes a brief look at the current loop connectors used by DEC.

The main focus of this post is the cabling needed to connect a VT220 terminal to a computer (such as a PDP-11/10) using the 20mA current loop interface.

DEC’s standard connector

DEC used an 8-pin Mate-N-Lok connector for all current-loop cabling between its computers and peripherals.

Here are some photos of the Mate-N-Lok connectors:

The female connector is AMP P/N 1-480459-0, DEC P/N 12-09340-01. Appears to be still available (minimum quantity 14) for US$0.751/ea.

The male connector is AMP P/N 1-480460-0, DEC P/N 12-09340-00. Appears to be still available (minimum quantity 15) for US$0.674/ea.

Full details of the pins are available at Page 155 of Tyco Electronics Catalog 82181 Revised 4-08.

The female pins are AMP/TE P/N 350078/61173 (wire gauge 30-22), 61314/60617 (wire gauge 24-18) or 61117/60619 (wire gauge 20-14).

The male pins are AMP/TE P/N 350079/61174 (wire gauge 30-22), 61116/60618 (wire gauge 24-18) or 61118/60620 (wire gauge 20-14).

Tyco Electronics Catalog 82181 says that Pin 350079 is good for 0.05 to 0.3mm^2, Pin 61116 is good for 0.2 to 0.8mm^2, and Pin 61118 is good for 0.5 to 2.0mm^2.

I measured a roll of common grey 4-conductor shielded data cable. Each conductor (and the drain wire for the shield) has 7 strands of 0.20mm diameter wire, so a total of 0.22mm^2 per conductor. Given that I do not intend to use wire that is thinner than this, I have decided to use the pins intended for wire gauge 24-18 (ie Female=61314/60617, Male=61116/60618).


On my PDP-11/10, the current loop connections come out of a PCB (DEC P/N M9970) which is plugged into a particular location in the computer backplane. The M9970 has a 40-pin male “Berg” header on it. This looks very similar to the 40-pin IDC headers that we commonly see on 0.05″ pitch ribbon cables today.

A short cable (approximately 3 feet long) plugs into the Berg header and has a female 8-pin Mate-N-Lok connector hanging off the other end of the cable. The cable is known by DEC P/N 70-08360, but is more commonly known as a BC05M cable. The same cable was also used on PDP-8 systems.

Here is a picture of the BC05M cable:

ASR33 Teletype

The ASR33 is a little different. It doesn’t have an external current loop connector on the ASR33 itself, or nearby. Instead it has a long (approximately 20 foot) cable that comes out of the back of the ASR33. One end of this cable is directly attached to the terminal strip and reader-enable board inside the ASR33. The other end has a male 8-pin Mate-N-Lok connector, which mates directly with the 8-pin female Mate-N-Lok connector hanging out the back of the PDP-11/10.

I won’t get into very much detail about the ASR33 current loop interface in this post, except to say that the current loop interface has one extra current loop called “reader enable”. The reader enable current loop enables the computer to request one frame (one byte) of data from the ASR33’s paper tape reader, and advance the paper tape by one frame.

The VT220 connector

Interestingly the VT220 also has an 8-pin female Mate-N-Lok connector on it. It’s on the back panel of the VT220, as shown in the picture below:

It wasn’t easy to find VT220 documentation relating to the current loop interface. The Technical Manual (DEC P/N EK-VT220-TM-001) doesn’t seem to show the pinout diagram. I was able to find the connector pinout in a hard copy of the VT220 Owner’s Manual (DEC P/N EK-VT220-UG-003).

Pin     Signal

 1      -12V

 2      Transmit -

 3      Receive -

 5      Transmit +

 7      Receive +

 8      Ground

Looking into the female 8-pin connector housing on the back panel of the VT220, Pin 1 is on the left and the two protruding plastic bumps (on the outside of the Mate-N-Lok female shell) are on the upper surface. Pin 8 is on the right. I confirmed this with a voltmeter by checking the -12V output pin.

I then put a voltmeter across the TX pair (pins 2 & 5) and confirmed the voltage was 0.0V. I repeated this on the RX pair (pins 3 & 7) and got the same result. This tells me that the VT220 is a “passive” device. This means that although it can sense the current flowing in the RX loop, and it can open/close the TX loop, it doesn’t actually provide a current source on either loop. So it requires that the device at the other end of the current loops (typically a PDP-8 or PDP-11/10 computer) will be the “active” device on the loop. Each current loop requires one (and only one) active device on the loop.

The “Transmit” and “Receive” designations are relative to the VT220. So “Transmit” means data being transmitted by the VT220 terminal to the computer.

DEC’s conventions for current loop connectors

From the information above I have been able to figure out the following about DEC’s conventions for current loop wiring:

  1. Each piece of DEC equipment (whether a computer, printer or terminal) would have an 8-pin female Mate-N-Lok connector on it
  2. A “null modem” cable, with male 8-pin Mate-N-Lok connectors on each end of it, would be used to connect any 2 pieces of equipment together
  3. You could save a few connectors (as the ASR33 did) by not putting a Mate-N-Lok connector on the device itself. Instead, these devices would run a longer cable (permanently attached to the device) direct to an 8-pin male Mate-N-Lok connector at the other end of the cable, which would mate directly with the 8-pin female Mate-N-Lok connector on the DEC equipment

The null-modem cable

To connect two devices that have 8-pin female Mate-N-Lok connectors we need a 4-conductor cable that “swaps” the TR and RX pairs, as shown below:

Active end (computer) Passive end (terminal)
5 (Transmit +) 7 (Receive +)
2 (Transmit -) 3 (Receive -)
7 (Receive +) 5 (Transmit +)
3 (Receive -) 2 (Transmit -)

Cables that are intended to connect a computer to an ASR33 need an additional two wires (for the reader enable current loop). However, it doesn’t make sense to include these in a null-modem cable, as they are unidirectional (the PDP-11 is always the transmitter, and the ASR33 is always the receiver). So don’t connect the reader-enable wires in a null-modem cable.