IIsi NuBus adapter being weird but only with one specific video card

[Arisotura]

I'm running into a weird issue with my IIsi NuBus adapter. Basically, if I put a Lapis ProColorServer II in it, the Mac doesn't boot -- it gets stuck on the gray pattern screen, doesn't do anything else, doesn't display any cursor or boot icon, doesn't try to access SCSI.

But, that's the thing.

If I put a Mac II high-res video card in the adapter, the Mac boots fine, and that card works fine too.

If I put the Lapis PCS in my IIfx, it boots and works fine.

I did a little test -- I added an interrupt switch to the IIsi, put in the Mac II HR video card, and went to the debugger to see if all the NuBus address space is read/writeable, and it is. What I wanted to see was if one of the NuBus transceivers was bad and causing these issues, but the Mac II HR video card would still work because it only uses the working lanes, but I'm not seeing any problems there.

Any ideas what might be causing this problem?

 

[Melkhior]

The IIsi power supply and internal circuitry isn't great, so my suggestion it to check the voltage on the 5V rail in all three cases (no video and both video device). I would expect the Mac to go into self-reset rather than just hang, but it's an easy check so worth doing. Ideally you want at least 4.85V in there at all time, if at any point it goes below 4.75 to 4.80 then the machine will misbehave.

 

[Arisotura]

The voltages seem good: 5.02V on 5V, 12 and -12 are also good.

However it seems that the +12V on the Lapis is 0...

Edit- doh, stupid problem. The +12V pin in the NuBus adapter's PDS connector was bent. The Lapis PCS needs +12V to generate its pixel clock, but I guess the HRVC doesn't use it and that was why it worked.

 

[Melkhior]

I wouldn't expect a video device to need 12V, very surprised it did for such a critical function... so it works now in the IIsi?

 

[Arisotura]

Yup, works absolutely fine!

 

[Bolle]

It’s not uncommon that voltage regulators on Nubus or PDS cards are fed by the 12V line.
DACs with external voltage references, high frequency clock generators that need their own 5V supply decoupled from the main 5V rail, ethernet cards with BNC usually also need 12V for the voltage regulator packs…

 

[Arisotura]

Yeah. In the case of the Lapis PCS, the Chrontel CH9201 clock generator needs 12V for its analog supply.