After assembling the board I programmed the EEPROM with a version of Assist09 with the ACIA address modified to reflect the new memory map. On power-up nothing happened. This isn’t entirely surprising but was worth a shot, it would have been great to see the prompt on my terminal. Assist09 uses a vector map placed in RAM, so to function correctly both memories need to be working correctly; I needed to see where the problem was. I considered jamming NOP instructions on the bus to check the processor was functioning, but the 6809 sets the address bus to $FFFF when executing instructions so it isn’t easy to see anything useful with this. Instead, I wrote a small piece of code that simply jumped to itself. This would exercise the EEPROM only, and it worked fine. The problem was therefore with the RAM.

I wrote another short piece of code that wrote to a fixed memory location in an endless loop, and I could see the correct data on the data bus synchronized with the control signals for RAM. This indicated that the problem was with reading, so another piece of code read from the same location in a loop. I could see straight away that when the read cycle started the RAM was disabled. I had just re-invented the write-only memory! Fortunately I was able to modify the circuit using the gates on the board (as seen in the photo) and both reading and writing worked correctly. That should be enough to get Assist09 running.

Reprogramming the EEPROM with the monitor still didn’t give me a prompt on the terminal. What could be wrong? I could see the ACIA being polled regularly so the code was running correctly, so what had changed? When I reset the board I could see that the ACIA transmitted something, but the terminal didn’t recognize it. I had switched from a 7.3728MHz crystal to a 3.6864MHz so expected the Baud rate to halve. What I realized later was that on the previous board the ACIA was clocked by the crystal frequency divided by 4, so my new design was actually running at twice the speed of the previous design. Changing the settings on the terminal program finally gave me the Assist09 start-up text and command prompt. With Assist09 running I can download code rather than reprogram the EEPROM so progress should be much easier.