Close

If It Ain't Broke, Don't Fix It

A project log for Novasaur CP/M TTL Retrocomputer

Retrocomputer built from TTL logic running CP/M with no CPU or ALU

alastair-hewittAlastair Hewitt 01/05/2020 at 18:190 Comments

There was one final delta between Rev. 1 and 2: The H-sync was put through the bus control latch to align it with the dot clock. This wasn't really necessary, so was rerouted to be a straight connection on the Rev. 2 board. This frees up a flip-flop for use elsewhere and was used to resample the output enable of the X register. However, this required an additional shift in the clock phase that was not made. The result was bus contention on the lower part of the RAM address.

It's surprising the board was able to run at all with this problem. The OTP ROM did not work because it contained the text fill code and this was crashing before the video loop could start. The problem was resolved by cutting a pin and using a patch wire to select the correct clock phase.

The Rev. 2 board is working and was able to run with the 35MHz dot clock. The quality of the video signal is greatly improved with the cleaner supply lines. The assumption was the cleaner supply would also improve the stability at 35MHz, but things are starting to glitch as they warm up. Dropping to 32MHz resolves any remaining stability issues and this is likely be the final dot clock speed.

There's not much more to test on the computer side of things, so testing is focusing on the new power supply design. A trip to the local electronic store to pick up more solder lead to a chance discovery. They had inexpensive linear power supplies. I don't need a regulated input and a big hunk of iron in the power supply has additional retro appeal. Another option is to add a bridge rectifier to the the board to support an AC input. This would only require a simple iron core transformer for the power source.

Discussions