Serial Interface

The serial interface is perhaps the only area of the maintenance manual that is somewhat brief in its description.  It covers the basics, but assumes you are running a 60ma current loop teletype / paper tape reader as the serial I/O device.  Most of us are not running teletypes, but serial ports connected to a terminal emulation program on a PC or a Mac computer.

For the record, the communications protocol required to load programs into the machine is: “8 data bits, two stop bits, None Parity”.  However, those programs that want to communicate with you, like Maindec diagnostics, Chess and others require seven (7) data bits, not eight.  This means changing the settings on your terminal emulator as you change between both functions.

The first thing one must do is convert the current loop from 60ma to 20ma otherwise you risk destroying any terminal or device you connect to the system that will be expecting 20ma.

60ma to 20ma current loop conversion

Thanks to David Gesswein who provided technical assistance with the following modification.  On the TTY card W050, replace the 750 ohm resistor connecting to -15v to 2.3K ohm.  This effectively reduces the current to the desired value.

The serial interface consists of six R220 flip flop cards, with timing generated by a clock located on the R450 card.  On my system the diodes on all six R220 cards were of poor quality and I had to replace all of them.  One thing to note however, on almost of the cards in the machine the diodes are all physically oriented in one direction. This makes bulk replacement relatively easy.  You just get the polarity right for the first diode, and the rest naturally follow.

However, they must have had the new guy design the R220 card because the diode orientation is all over the place.  Getting one wrong can chew up a lot of troubleshooting time, as can a careless solder splash….

The second requirement is getting the timing right on the R450 card.  This is not mentioned in the manual, and I have discovered subsequently that various cards were used for the timing process in the machine.  The R450 however has four pots that need to be correctly adjusted to deliver:

  1. The correct baud / bit rate of 110 bps for send and receive, and
  2. The correct mark space ratio.

Get either of those wrong and data corruption will follow.

I tried a couple of current loop to RS232 current loop converters before I finalised on this one from

It’s a great unit and has worked flawlessly.  What’s more they ship free to New Zealand.  This unit connects into any standard USB-RS232 cable.