Using a new new de-soldering tool that I've acquired, I've been able to de-solder and remove the old UV EPROM 27HC256 inside the main internal display and replace it with a socket. This has allowed me to install a W27C512 Electrically Erasable PROM. I've done this so then it will be easily reprogrammable and to keep it running for longer.
The new EEPROM is actually 512K instead of the original 256K and uses an extra address line pin due to this, however the pinout is identical apart from the VPP pin being changed for this new address pin. The VPP pin, which is used for programming, is constantly tied to the 5V. The result of this is that when the CPU tries to access data, it will always read the second 256K half.
A simple fix for this is creating a blank 256K header before the program data. The normal address pins act like normal but shifted by 256K in the data due to the constantly high A15 pin. But because the data is now shifted by 256K it negates this effect.
Here you can see I've changed the startup message from the original "BRB Class 465".
Using the checksum calculation method as previously stated, I also recalculated the checksum to avoid a startup failure.
And success!
