On the 40th Anniversary of the ZX81 I retrieved mine from the shelf in my office and plugged it in anticipating some old skool gaming…….. not going to happen 🙁 . The prompt was displayed, but nothing happening when I pressed the keyboard. I tried another keyboard but still no joy. After some poking around with the oscilloscope I decided that it might be the RAM.
Having previously built memory testers for Spectrum DRAM chips, I decided to create one for the 2114 SRAM chips used by the ZX81.
The 2114 SRAM chips are much easier to use than the 4116 DRAM chips used by the lower ram in the Spectrum. Only one voltage, 5 volts, and no RAM refresh cycle to worry about being static rather than dynamic RAM. Happy days…
Creating a prototype on a breadboard didn’t take long. I used an Arduino Nano to run the test suite and drive the RAM chip under test. I designed a number of tests that performed various bit rotations across the memory range.
The prototype worked well, running sets of tests over the target 2114 chip.
Following the success of my Z80 processor tester I decided to create a PCB.
So far so good. I intend to send the design off to china for manufacture next time I have some other boards to be made… I’ll update when this is done.
All code and PCB designs are here on GitHub
Addendum : I was able to determine that the ZX81 RAM was fine using the prototype 🙁