ICL OPD

[stephen_usher]

After two nights' work I now have (bar the decoupling caps and pull-ups) a complete schematic of the Computer One floppy disk interface.

I found that the ROM pack pinout as posted in an earlier post needs amending. Pin 27 (marked as R/W) is "PACK_RW" (I can't remember the sense off hand) and is an output and Pin 28 (marked as PAL) is "HOST_RW", which is an input (as seen by the ROM pack).

[Pr0f]

I noticed that in the zips - one the ROM chip images is all blank. I also have a white written label in my IC7 position - which would be the "D" ROM.

I've posted it here so you can see how the contents compare:

[stephen_usher]

Thanks.

[stephen_usher]

Here's an version of the opd-decode.pdf with the ROM module pin-out updated from what I've learnt from the Computer One floppy interface.

[stephen_usher]

First look at my Computer One floppy interface recreation...

PCB order going in to PCBWay this afternoon.

Here's the schematic:

Also: There was an error in my modified version of the decoding PDF. Pin 29 raises an interrupt.

[stephen_usher]

P.S. I'm not sure if that clock circuit using the 74LS04 is going to work. *crosses fingers*

[Chr$]

Very interested to hear if it works as I'd like one too.

[stephen_usher]

Well, this is just the test version to verify my reverse engineering.

With the Mk.II I think I'll split the WD1772, clock and FDC logic onto a daughter board connected via a ribbon cable. Not only will it make the board plugging into the ROMpack smaller but it will mean that in future I can develop a board to replace the WD1772 with an emulator and SD storage, probably using a RPi Pico. The Mk.II will have some of the logic, such as the address decode, put into a GAL to reduce the chip count.

[Pr0f]

Can anyone think of a reason why the PicoVGA interface would not work with the ICL OPD ?

I just bought an early Christmas present of one of these on eBay - as the original unit I had was just the boards, and I had the keyboard from long ago. I plan on making up a small unit with a switched mode PSU and the PicoVGA board so that the device can be used with a VGA screen

[stephen_usher]

You'll have to invert the RGB and sync signals from the OPD, but I think you should just be able to pass them through a 74LS04 to do that before sending them to the interface.