UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Gemini 80-Bus System

NealCrook · 25th Jun 2019, 10:50 pm

Hi John,
I think Julie's suggestion of a custom EPROM is a good one. The best Z80 debug tip I ever got was from someone at NASCOM when my '2 stopped working. He told me to hold RESET low and then follow the address bus from the CPU through address decode to the ROM, and follow the data (0x31 naturally) back from the ROM to the CPU. My test equipment was a TIL209 LED and a 270R resistor.

When I brought my '2 back to life last year, I used a similar technique (though with my 'scope instead of the LED)... turned out to be a bad socket, causing an open-circuit which a TTL gate was considering to be a 1, causing the address bus to skip. I'd almost forgotten about those TTL behaviours.

With some very short custom EPROMS you can toggle all of the address and data bits, do I/O writes to the other devices on the bus. If you just do (eg) 1 I/O write in your loop it will be easy to trigger a scope or analyser and work your way along the bus. Sounds like a fun project for a rainy afternoon (Hmm. Weather's getting hotter..)

regards,
Neal.

25th Jun 2019, 10:50 pm	#124
NealCrook Tetrode Join Date: May 2019 Location: Reading, Berkshire, UK. Posts: 51	Re: Gemini 80-Bus System Hi John, I think Julie's suggestion of a custom EPROM is a good one. The best Z80 debug tip I ever got was from someone at NASCOM when my '2 stopped working. He told me to hold RESET low and then follow the address bus from the CPU through address decode to the ROM, and follow the data (0x31 naturally) back from the ROM to the CPU. My test equipment was a TIL209 LED and a 270R resistor. When I brought my '2 back to life last year, I used a similar technique (though with my 'scope instead of the LED)... turned out to be a bad socket, causing an open-circuit which a TTL gate was considering to be a 1, causing the address bus to skip. I'd almost forgotten about those TTL behaviours. With some very short custom EPROMS you can toggle all of the address and data bits, do I/O writes to the other devices on the bus. If you just do (eg) 1 I/O write in your loop it will be easy to trigger a scope or analyser and work your way along the bus. Sounds like a fun project for a rainy afternoon (Hmm. Weather's getting hotter..) regards, Neal.