Nama
Member
Hi all,
Been poking around on and off with this one for a while now.
I've got it to the point where it can recognise the test ROM.
The test ROM identified a few RAM chips that need to be replace...last RAM to get replaced was RAM #7, however it still says that RAM #7 needs replacing! I've moved the RAM around, but always #7 is said to be bad.
On the test ROM screen there are a repeating pattern of thin lines. if a RAM is bad, a line will be missing or double thickness. The pattern of lines looks perfect and complete on my system, suggesting that the RAM is actually ok. However next to the number '7' seems to be what looks kind of like a 'c' shape. I don't know what this is exactly, and I haven't seen anything similar in my internet searches. If I temporarily remove other RAM chips (the ones that I've socketed), the bad RAM number will change, but the 'c' shape remains.
Finally I have some observations from probing the circuit with a logic probe.
With he test ROM installed, address lines A11, A14 and A15 (measured from the 8080A CPU) do not seem to be pulsing and are low.
For the 74LS174 @ D7, the input data lines seem to all be pulsing, but the outputs on pins 2, 7, and 12 are all low. Only the outputs on pins 5, 10 and 15 are pulsing. However Pin 10 seems to be pulsing at regular intervals, like a clock. I've looked at this pin with my (very old) scope, and it does seem to be pulsing regularly.
Anyway, hoping someone may have an insight into my issue. I really thing I'm very close to getting this all working, and if I can solve this last issue I'll be up and running.
Much thanks in advance
Phil
***UPDATE*** Just wanted to add that I have also tried this with my Braze Multigame kit, and it also indicates a problem with RAM #7. Probing the address lines of the 8080 with the Braze installed now shows activity on A11, A14 and A15 (A15 only pulses for a short time after reset)
Been poking around on and off with this one for a while now.
I've got it to the point where it can recognise the test ROM.
The test ROM identified a few RAM chips that need to be replace...last RAM to get replaced was RAM #7, however it still says that RAM #7 needs replacing! I've moved the RAM around, but always #7 is said to be bad.
On the test ROM screen there are a repeating pattern of thin lines. if a RAM is bad, a line will be missing or double thickness. The pattern of lines looks perfect and complete on my system, suggesting that the RAM is actually ok. However next to the number '7' seems to be what looks kind of like a 'c' shape. I don't know what this is exactly, and I haven't seen anything similar in my internet searches. If I temporarily remove other RAM chips (the ones that I've socketed), the bad RAM number will change, but the 'c' shape remains.
Finally I have some observations from probing the circuit with a logic probe.
With he test ROM installed, address lines A11, A14 and A15 (measured from the 8080A CPU) do not seem to be pulsing and are low.
For the 74LS174 @ D7, the input data lines seem to all be pulsing, but the outputs on pins 2, 7, and 12 are all low. Only the outputs on pins 5, 10 and 15 are pulsing. However Pin 10 seems to be pulsing at regular intervals, like a clock. I've looked at this pin with my (very old) scope, and it does seem to be pulsing regularly.
Anyway, hoping someone may have an insight into my issue. I really thing I'm very close to getting this all working, and if I can solve this last issue I'll be up and running.
Much thanks in advance
Phil
***UPDATE*** Just wanted to add that I have also tried this with my Braze Multigame kit, and it also indicates a problem with RAM #7. Probing the address lines of the 8080 with the Braze installed now shows activity on A11, A14 and A15 (A15 only pulses for a short time after reset)
Attachments
Last edited: