building my own relay computer

Tom, any reason for choosing the Harvard architecture? I'm currently in the middle of a relay computer build so I'll toss in my 2 cents. The most important thing I hit on early is to define the instruction set, work out a number of potential applications/uses on paper (or excel), and step through each instruction to make sure the machine is capable enough. You'll probably discover a number of efficiency improvements you can make to the hardware while doing so, and a few "hardware subroutines" worth adding. You can also baseline how long a program will take to run if you settle on a clock rate, and determine if there's a better way to do it. Ultimately, relay computers are SLOW. Don't take 6 cycles to do what could be accomplished with a couple extra relays and a single pulse. Well thought out HW and instructions will pay big dividends when the soldering iron comes out. -Cory