It took an hour or so for somebody to reply with a suggestion, and they were spot on.
Took me ten minutes with a scalpel and soldering iron to cut and bridge a couple of tracks and hey-presto, it sprang into life and works perfectly.
So, I now have a fully functional System 1 CPU board and keypad/display.
There are some updates I need to do to the PCB foils and the schematics to reflect the fix and add some additional ideas I've thought of, and then I'll put all the design files up on this site for those that want to build their own.
For those that know the Acorn System 1, there are a few changes in my version that should make building this one a little easier, but will also make it not completely compatible with existing Acorn hardware setups.
Whilst it does make use of the old bubble style LED display, there is provision to add a larger LED display as I've routed power to the display connector. I will have a design for a powered LED display soon.
It makes use of the W65C02 CPU instead of the older 6502. This gives some additional features including the ability to single step the CPU for debugging, and it will run at speeds much faster than the 6502 which was limited to 1 or 2MHz depending. Actually it will run at any speed from DC to around 14MHz in fact and it's useful to be able to run it at a very slow speed for debugging.
The edgeway connector is different to the original. Mine uses a 96 position Euro connector with columns A+C loaded giving 64 pins, and whilst I did generally follow the original Acorn layout there are a couple of differences and some additional pins used. This means that the board probably won't be plug-and-play into an original Acorn existing rack system or be pin compatible with Acorn expansion boards.
There is a more flexible clock oscillator circuit which can make use of any crystal up to 16MHz and divide down to the 1MHz required if you want full compatibility. This will save the having to source an expensive 1MHz crystal; assuming you can find one.
Instead of the two small PROMS on the original, only the EPROM socket is provided and this has been modified to accept the more common and simpler to program 2764 instead of the 2716. Many of the cheap EPROM programmers won't program 2716's. It will also work with an 2764 EEPROM.
The 2 x 2114 RAM chips have been replaced with a single 6116 RAM chip.
This also gives you 2K instead of 1K of RAM (this is configurable with a jumper).
(I will be changing this to use a more common RAM chip.)
The next step is to find a suitable workaround for the last component that is really difficult to find; the INS8514. I'm lucky in that I have one but they are really hard to find and very expensive. They are also quite easy to damage and without one, you can't get the display or keypad working. I've got some ideas for this.
The cassette interface has been removed. There is provision for a plug in card to be added to the keypad as all the control signals are still present if you really do want a cassette interface, but I think in these modern times, we can do better than magnetic tape. That's a future project.
The main PCB has been modified to take a standard euro PCB connector for mounting in a rack. Also, the keypad/display is now connected via a 20 way ribbon connector with ICD connectors at each end. This means you can remove the keypad if desired without having to desolder/resolder the ribbon cable.
There's also a power on LED on the main PCB and provision for a reset switch. The keypad also has a reset switch and also has the IRQ and NMI buttons.
As long as it's set to a 1MHz clock rate, none of the above changes should break compatibility with an original System 1; with the exception of the cassette interface and that can be added if required, so it should be able to run all the original software.
As with the original there are no surface mount components and provision is made to socket all the ICs. Whilst there is a fair amount of soldering required to assemble one of these, it should be relatively easy for some somebody with only limited soldering experience. The hardest part is probably making the 20-way keypad ribbon cable.
Once the plans are fully updated and I've checked to make sure the new PCB designs work as expected, I'll post something here in my blog.
I'd started to think I would never get this working so I've really very very pleased it's now running as expected.