I’ve been working on building a computer based on the DIYCalculator (http://www.diycalculator.com/) as time allows. There’s a virtual version of this computer already in existence that was written by the authors, but I wanted a real physical one to play with.

Figure 1. Physical version of the DIY Calculator.


Figure 2. DIY Calculator and another one during assembly.

"Shhhh… the second machine is Max’s birthday present if I ever get time to complete it, so don’t tell him."

 I’ve also built a DIY baby sister as well; I had a day spare and wanted an excuse to play with some new SSRAM’s.

Figure 3. BabyCalc hooked up to a firmware programmer.

Anyway, one thing I’ve been desperate to find is an old teleprinter with a paper tape punch and reader. These are rather hard to come by these days after being replaced by VDU’s, and the few that are remaining tend to be in the hands of collectors and museums. However, one flavour of teleprinter that is sometimes still available is the telex machine and you can often find the odd one on EBay.

I’ve already previously found an old telex machine but it looks like I found it 10 years too late and it was in a very sorry condition and it wasn’t practical to repair it.

As look would have it, I came across, bid and won another telex machine. It was a drive half way across the country to collect it mind, but it got us out of the house for a day; Barnsley is lovely this time of year.

 
Figure 4. Creed 444 teleprinter.

So, I’m now the proud owner of a Creed 444 telex machine that looks to be in pristine condition. It even came with a roll of telex paper, blank paper tape and an ink ribbon. What it didn’t come with were any cables.

The plan is to use to teleprinter as an interface to my DIY computer; commands are input via the keyboard and responses printed out on the printer mechanism.

Unfortunately, I don’t think I will be able to use the terminal to write programs on, as telex machines don’t use a full 8-bit ASCII set; they are only 5-bit and this limits them to 32 possible codes. They manage to get more than 32 printable characters by sending code sequences to switch from alpha characters to numerics, but the actual printer mechanism is a series of metal hammers as found in an old typewriter, so you can only print the characters that are physically available, but once I get it working it will do for my needs. The hope is to be able to store programs, and then load them back from paper tape when everything is complete. Again, the paper tape reader / punch is only 5 bits but I can get around that simply enough by splitting a byte into two characters of 4 bits each or some other scheme.

The first challenge is to get the teleprinter to actually work. I said that it didn’t come with any cables, including no power cable.

It was obvious where the power cable enters the unit, but it’s of a very unusually style and so I’m not hopeful of ever finding a suitable connector. Undaunted, I removed that part of the chassis and directly soldered in a power cable, plugged it in and switched on, and it seems to run lovely.

I say seems because whilst it makes a heck of a racket, and you can smell the fact that it’s been freshly oiled, it doesn’t actually do much. Pressing the keys and other buttons doesn’t do a think; you can just hear the main drive motor spinning.

A lot of digging around on the net revealed the problem. To make it do anything useful it’s going to need an 80v DC power supply.

So, time permitting at the weekend, that’s next on the list of things to do.