Building a USB Serial to 5 Pin Domino DIN Cable

 The BBC Micro computer uses an almost extinct 5 Pin "Domino" DIN connector for its RS423 (RS232 compatible) serial port. I wanted a neat cable to connect my BBC computers to things like SerialFS and TCPSer but there wasn't really anything on the market that was neat and compact.

I needed a USB to RS232 level cable, but without a DB9 on the end, as the USB Serial chip is often inside it. Many cables available online are TTL level which makes them unsuitable. After a bit of hunting I discovered the programming cable for the Kenwood TM-D710 Radio uses RS232 levels and Tx, Rx, Gnd, RTS and CTS which is exactly what we need, so I set about modifying it.

I acquired the cable for £19 from Amazon but other USB programming cables for the "Kenwood TM-D710" radio may also be suitable. I already had a 5 Pin Domino in the parts box but they're available on eBay.

As the cable colours don't follow any particular standard I had to "bleep" them out with a multimeter. To do this I cut the mini DIN off the cable leaving a couple of inches cable, and stripped each conductor. I then continuity tested each pin to match it to its colour.

Here's the pinout of the supplied 8 PIN Mini DIN. Note its as you look into the connector, which is different to some conventions but makes it easier as you'll be looking it from the front as you bleep it out.

These are the colours mine was using. Note it may very between cables so you should always check, but this might be a useful reference

With the colour mapping in hand, you can now solder it up to the 5 pin domino like so:

That should give you a neat USB cable that works with your BBC computers.