Relay Computer

My journey in constructing an 8 bit relay computer from scratch ... made from of a ton of solder, wires, lights, relays, sweat, tears, swearing and money.

Incrementer Construction (Part 2 of 2)

17 Dec 2017    

In my last post I covered the construction of the lower incrementer card which is effectively a 16-bit half adder. This time I’m going to cover constructing the upper card of the incrementer which is a 16-bit register that can hold on to the incremented value before pushing it back out to the address bus.

The backplane connectors and LEDs were soldered down in my last post but there’s plenty more soldering to do. First up I can add the relay sockets, wire wrap posts and card interconnect wrap posts:

Upper incrementer card with added sockets and wire wrap posts

This time around I haven’t soldered on the stacking header that connects the two cards together just yet as I’ve found it gets in the way when wire wrapping or can be damaged if I’m not careful. I’ll solder this on just before I bring the two cards together to form the incrementer unit as a whole.

Next up I can solder down the ground and power rails on the back of the card:

Upper incrementer card with added ground and power rails

Nothing too unusual here besides there being quite a lot of relays on this card and consequently a lot of ground rails. So, soldering done (for now) … on to the wire wrap …

Upper incrementer card with added internal wire wrap links Upper incrementer card with added power wrap Upper incrementer card with added diodes Upper incrementer card with added outgoing address bus wrap Upper incrementer card with added incoming incremented value wrap Upper incrementer card with added control wire wrap

Going through the pictures in order (top left to bottom right) we have: the internal register relay bits wiring; the power supply wiring; diodes added; the outbound address bus wiring via the gating relays; the inbound incremented value wiring (again, via gating relays) and finally the control wiring.

With the wire wrap done the card is pretty much complete - just needs the relays popping in …

Completed upper incrementer card (front side) Completed upper incrementer card (rear side)

… and that’s it for the upper incrementer card. You might have noticed that this post has perhaps been a little brief and that’s because there’s not much to show that hasn’t been seen elsewhere before. Partly that’s because this is effectively just another register but also because I’m pretty settled on the construction method I’m using. One slight slip up on this card though is the use of diodes on the outbound address bus coming off the register bit relays. They’re not actually necessary as there’s no risk of backfeed messing up the register’s value and it’s come about because I’ve copy/pasted the design from another register and tweaked accordingly. In practice it won’t cause a problem just meant the card was slightly more complex than it needed to be.

In my next post I’ll sandwich the upper and lower cards together and then give the whole incrementer unit a test.