Talking to the IPC: How do you do it from low level?

[Pr0f]

For the IPC > ZX8302 the communication is similar:

;
; Send 8 bits
;
IPC_Send_8:
mov r5,#008H
jmp L0358
;
; Send 4 bits
;
IPC_Send_4:
swap a
mov r5,#004H
L0358:
rl a
mov r6,a
jb0 L0369
L035C:
in a,p2
jb7 L035C
movx @r0,a
anl p2,#07FH
movx @r0,a
orl p2,#080H
mov a,r6
djnz r5,L0358
ret
;
L0369:
in a,p2
jb7 L0369
movx @r0,a
orl p2,#080H
movx @r0,a
mov a,r6
djnz r5,L0358
ret

R5 is used as a loop counter - doing either 4 or 8 passes. At L035C - we read P2 port looking for it to drop to 0 - in other words - we can't transmit any bits until the ZX8302 tells us by dropping COMDATA to 0.

Depending on whether the bit to be sent is a 0 or 1:

If a 0 is to be sent :

As soon as we see COMDATA go to 0 we pulse /WR line low on the IPC (bit 6 of the port for reading IPC (COMCTL), and then perform a literal AND on P2 (preserve other values on the port but reset bit 7 - this sends a 0 on COMMDATA and we send another /WR pulse (COMCTL) and then set COMDATA back to 1 and loop around for the next bit

If a 1 is to be sent:

As soon as we see COMDATA go to 0 we pulse /WR line low on the IPC (bit 6 of the port for reading IPC (COMCTL), and then perform a literal OR on P2 (preserve other values on the port but set bit 7 - this sends a 1 on COMMDATA and we send another /WR pulse (COMCTL) and loop around for the next bit

So in both cases for writing bits - COMCTL will pulse low twice - once before and once after the bit sent, and in both cases we wait for COMDATA to go low before we send anything, leaving it high after sending

[Pr0f]

Ah - I see someone replied in the time it was taking me to write my reply - lol.

[stephen_usher]

Looks like, at least on emulators (qlay3, which has IPC emulation), you don't have to send tree bits per bit, just one at a time.

I managed to get the RESET command working but selftest doesn't seem to want to take an argument to echo, but that may be an emulator bug as I doubt anyone tested that.

I've had to put the diag ROM project on hold now as I have to build a model railway for RetroFest2025 (Steam, in Swindon, 31st May, 1st June) and I only have four and a bit weeks! (*gulp*)