I had a look at reducing the amount of area for Trinity. I came up with a significant saving by reducing the number of levels 'pages' dedicated to the Exception Level. Bringing this down to four pages means that at the fourth nested level you need to push stuff into an Exception Stack. I could make this more automatic but I'm going to leave it for now.
This means that everything is back in play.

Back to getting the RS232 to work. A bit of pipecleaning. I have an old Amstrad NC100 (Noddy) which happens to have an RS232 port. Resurrecting old kit is fun but it appears to be working.
I bought what is advertised as a USB to RS232 conversion cable that can cope with a full +/- 12 V swing. This along with a Null Modem cable I connected up to Noddy and the Mac. Using Zterm I managed to get comms up and running in both directions at 9600 Baud. The following steps will now be made

1. Send a series of characters to Noddy from the FPGA board at 9600 Baud using the Null Modem cable.

2. Write a routine that runs on an interrupt to take the incoming character and then display it on the LCD display.

3. Send characters to the FPGA board from Noddy at 9600 Baud again using the Null Modem cable.

4. Connect the Mac to the FPGA board using the USB RS232 cable and the Null Modem.

5. Expand the amount of Memory available to the Trinity Dual Complex.

6. Write some kind of Monitor to allow downloading of programs and uploading of results.