You should be seeing different pairs of signals on C0 and C1 depending on bitmap screen size. I think C0=0/C1=0 is mode 3 (16 KB), C0=1/C1=0 is mode 6 (8 KB), C0=0/C1=1 is mode 0/1/2 (20 KB) and C0=1/C1=1 is mode 4/5 (10 KB), but don't quote me on that. The two C1 and C0 outputs go to IC31, a custom IC I think, so a bit of a black box. But probably worth checking the signals are connected up correctly.
If bits 0 and 3 are never low simultaneously, and yet keys are causing sounds, worth checking both bits are wired up as expected to their destinations. Sound and keyboard both use SA0...SA7 as their data bus, but the OS programs things so that the keyboard is never driving the bus while the sound is reading it. But if you're getting audible results from pressing keys then clearly that's not happening.
Since you were a bit uncertain about IC10 already, if everything looks otherwise in order, it wouldn't hurt to replace it? If you've got a working BBC B, maybe you can use its IC32?
--Tom
If bits 0 and 3 are never low simultaneously, and yet keys are causing sounds, worth checking both bits are wired up as expected to their destinations. Sound and keyboard both use SA0...SA7 as their data bus, but the OS programs things so that the keyboard is never driving the bus while the sound is reading it. But if you're getting audible results from pressing keys then clearly that's not happening.
Since you were a bit uncertain about IC10 already, if everything looks otherwise in order, it wouldn't hurt to replace it? If you've got a working BBC B, maybe you can use its IC32?
--Tom
Statistics: Posted by tom_seddon — Tue Feb 04, 2025 1:00 am