I wonder if we could use a 74HC93 for my earlier idea of using a counter to generate the volume commands.
It has a 'master reset' which we could connect to the ISA line I suppose. This will force the counter to 0 whenever the system is reset.
Then you can let it count up with an edge-triggered signal.
It has a 4-bit output, which we could use to generate the two bits we want, and a 'carry'.
We can use the inverted carry to generate the 'fixed' bits in the 1xx11111 pattern we are looking for. That would turn them into 0xx00000 once we counted to 100.
xx will be taken from the first 2 bits of the counter output directly.
That part would take care of the actual commands.
When there is no 'carry', we are in 'init' mode, so we block the ISA bus (with some and ports?).
Once the 'carry' goes high, the circuit outputs 00000000, and we just want to ignore it, and unblock the ISA bus.
Then what we would still need, is to do the actual handshaking with the SN76489 to process the data, then pulse the counter to increment by one, and process the next data.
I don't understand the exact communication between the SN76489 and the ISA bus... but I suppose it is something along the lines of us having to signal that we want to write to the chip... the chip then signals that we can (which we can ignore I suppose), and it will take the data from the 8 input pins, and then signal ready?
In which case, could we 'hardwire' the write signal with the inverted carry, and use the ready signal to pulse the counter?
Probably needs some finetuning, but... is there any merit to this idea?
It'd be clean and simple, and wouldn't require any programming of ROMs or such.