Trying to reverse engineer the William Stuart Systems ZX81 sound card.

[stephen_usher]

I think that I've worked it out:

BC1 = WR and !A2 and I/O request
BC2 = I/O request and !A3 and !A4
A8 = BC2

A9 low if A0, A1, A6, A7 high, A5 low

110xxx11 = base 195

During I/O request.

BC1 high if write and A2 low
BC2 high if A3 and A4 low, low if either A3 or A4 high

BC1 high addresses 195/203/211/219
BC2 high addresses 195/199

Port 195 = BC1 high, BC2 high
Port 199 = BC1 low, BC2 high
Port 203 = BC1 high, BC2 low
Port 211 = BC1 high, BC2 low
Port 219 = BC1 high, BC2 low

Knowing this, the only useful port numbers, given that BC2 has to be high otherwise the AY-3-8910 is high impedance, are ports 195 and 199.

The truth table for the AY is:

Code: Select all

BDIR		0		0		1		1
BC1		0		1		0		1
BC2		1		1		1		1
		Inactive	Read		Write		Latch register address
Port		IN 195		IN 199		OUT 195		OUT 199

Could someone sanity check this please.

[stephen_usher]

On another site I was directed to this: https://logic.ly/demo/ which is really useful!

It seems that I did get the logic correct after all.