Optoisolator for vintage digital current loop serial interface

Question

I'm designing an adapter for a vintage 20mA current loop interface, transmitting serial at 2400 baud. It's for Digital Equipment computers from the 1970s and 1980s, for a museum. We don't need to a general solution, just one for this particular board: a DL11-A Asynchronous Serial Line Interface. Our primary goal is of course to protect the museum pieces, which are from about 1978 onwards.

Solution

My idea is to use a simple 4N35 (datasheet) optoisolator circuit, with current limit resistor, for both input and output, with complete separation of power supplies. EDIT to add: the cable is at most 20 or 30 cm, more likely 5 cm.

The question: Do I need anything more sophisticated?

See below for details of what we're interfacing to and my simulations.

Circuits we're interfacing to

The input and output circuits are as follows

• The input comes in at labels K (+5V) and S (-15V) and comes out at H which feeds a 7413 Schmitt gate with 10K pullup at E.

• The output is a PNP pull-up to +5V on AA and -15V on KK.

• A D664 diode is apparently like a 1N3606 or 1N4153 (datasheet)

• 6534D transistor is like MPS6534 (datasheet)

Edited from DEC manufacturing drawing microfiche, PDF page 9

Own work

Simulations

I wasn't sure how to simulate these exactly (especially the optocouplers), but nevertheless this is what I tried. In the input it looks like R102 can be anywhere from about 1K to 5K and still get good TTL levels at E. In the output, I selected R201 as 1K to get about 10 mA across D201.

^{simulate this circuit – Schematic created using CircuitLab}

^{simulate this circuit}

Answer 1

Looks fine to me. I would put a Zener between the C and E of the phototransistor. The transistor would be bypassed when the open circuit voltage reaches the Zener voltage, or when the lines are reversed.

For maximum flexibility, I would put the LED and the phototransistor inside of a rectifier bridge. That's the retro equivalent of MDIX in Ethernet :)

The rectifier diodes can also be small Schottkys.

^{simulate this circuit – Schematic created using CircuitLab}