They are used. U42-18 connects to 1J21-12 and U42-19 connects to 1J21-13. Those signals continue to 10J4-12 and 10J4-13. 10J4-12 connects to U12-19 and 10J4-13 connects to U12-18. See images below.

They are pulled up to VCC to prevent floating signals.

01_cpu_u42.jpg

Have you tried reversing the ribbon cable (reverse both ends)? Have you tried a new ribbon cable? The signals on the sound board side are also pulled up to VCC to prevent floating signals.

02_snd_u12.jpg

You need to be more specific. CB1 and CB2 are not specific enough. U42-CB1 connects to U12-CB2. U42-CB2 connects to U13-CB1. So which CB1 and CB2 are you referring to?

CA1 and CB1 are input signals to the PIA. CA2 and CB2 are typically used as output signals from the PIA.

The software on the main board programs the PIA with the appropriate 8-bit data in U42-PB0 to U42-PB7 and then uses U42-CB2 to send the data to the sound board.

When the sound board PIA receives the signal at U12-CB1 it reads the data into U12-PB0 to U12-PB7 and performs the appropriate action. The PIA transfers the 8-bit data to the software on the sound board.

It's likely the sound board sends a "sound complete" signal back to the main board via U12-CB2 to 10J4-12 to 1J21-12 to U42-CB1.

You will likely need to study the schematic a little more for a better understanding.

You are both correct. I quoted from the much more common System 11A/B/C sound board. I have much more exposure to this board and I gravitate to the more common scenario but this was an incorrect assumption. In my defense it was 1:30am local time for me and I probably should have gone to sleep instead. The main board to sound board interface did not change from the original System 11 release to the final System 11 release. The available signals were fixed when the board was first manufactured. The software protocol might have slightly changed but it's unlikely that it did. There are, in essence, 8 data bits and 6 software related bits and other processor related signals. I do have a spare Pinbot sound board that is still in the shipping box that it arrived in. No time to process it - or even look at it.

That is probably the reason why Williams added the pullup resistor SIP to the later sound board - to prevent floating signals.

I assume the software only programs the PIA and sound board control signal when in music test. You didn't specify if you were in music test while taking these measurements. I am also not 100% sure but it might be possible that the control signal state is changed faster than your logic probe can detect. An oscilloscope should be able to verify this. I have not done this myself so that is not gospel.

These components are known to fail. The SIPs above U42 are often damaged from alkaline corrosion. Any or all of the five can be damaged. You can measure the resistance as verification. It's a simple test. Be aware though that you are measuring them in circuit.

This is generated by the reset signal from the main board reaching the sound board, correctly executing software on the sound board, the 1408 DAC, the analog processing, the power amplification and the speaker wiring all working.