Posted By
crock
on 2021-01-26
06:03:12
 |  Re: C16 Black Screen, DIAG264 flashing
Hi Hubi, I created Diag264 so hopefully I can help.
The documentation at https://www.inchocks.co.uk/commodore/Diag264/ does explain what the flashes mean. To quote from the "LOW RAM TEST" section:
"The screen border will cycle through a sequence of colours as each test byte is used and, as the low-ram area includes the screen and colour memory, you will see the contents of the screen rapidly change. The same reporting mechanism is used if an error is found during this test, with the screen background being black. If a failure occurs at this point, the number of screen flashes will point very strongly to the defective DRAM. The table below shows which IC is the likely problem."
I tried to copy the table here but the forum software messes up the formatting by removing extra whitespace.
Flashes Bit Plus/4 IC C16 IC C116 IC
1 D0 U11 U5 U5
2 D1 U12 U5 U5
3 D2 U13 U5 U5
4 D3 U14 U5 U5
5 D4 U15 U6 U6
6 D5 U16 U6 U6
7 D6 U17 U6 U6
8 D7 U18 U6 U6
If you read down the table you can see that 5 flashes suggests a problem with bit 4 of the databus, which in a C16 is implemented by the RAM chip at U6. However, the fact that the same same CPU in another machine also gives 5 flashes is a bit suspicious as it would be a bit of a coincidence (but not impossible) for the same RAM chip in 2 separate machines to be at fault.
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