ROM upgrade

In this page, the procedure to convert your ZX80 to a ZX81 that operates in FAST mode, will be described. This procedure does not require any modification to the ZX80 PCB. It just requires replacing the EPROM with another one, no cut-traces, no PCB soldering at all. Furthermore, you may return back to your original ZX80 at any time, by just fit in the ZX80 EPROM back again. Here, I will borrow some useful information from Paul Farrow's website.

One of the main limitations of the ZX80 was its lack of support for floating point arithmetic. This was resolved when Sinclair released an upgrade ROM for the computer. The ROM was a drop in replacement and had a capacity of 8K compared to the original ROM's 4K. Fortunately, the ZX80's circuit had been designed to support either a 4K or 8K ROM. Machines were then referred to as either having a 4K or 8K ROM or as supporting 4K or 8K BASIC.

The 8K BASIC ROM was identical to that used in the newly launched ZX81 and so it was designed to be backwards compatible with the ZX80 hardware which lack the functionality to support a steady display. A ZX80 fitted with the 8K ROM effectively operates just like a ZX81 that is permanently in FAST mode.

The 8K ROM BASIC was not a complete superset of the 4K ROM BASIC. It lacked the TL$ function and generated different tab positions when using commas in PRINT statements. For more information on compatibility issues visit Paul Farrow's website. Many 4K ROM BASIC programs utilized the system variables to locate the position of the display file, etc, thereby producing further incompatibility. As a result, it is generally not possible to load 4K ROM BASIC programs into a machine fitted with the 8K BASIC ROM.

To upgrade the ZX80 to support the 8K BASIC requires replacing the 4K ROM with the 8K ROM. Note that the 8K ROM requires changing the keyboard layout too. The only 8K EPROM that is pin-compatible with the ZX80 is the MCM68764. Note that the MCM68764 EPROM requires a programming voltage of 25V, which many modern day programmers do not support. Programming this EPROM can be tricky, and I am not going to cover this procedure here. The real ZX81 uses a masked 2364 ROM.

Reverting a ZX80 back to its original 4K BASIC status is simply a matter of re-inserting the 4K EPROM.  Note that the 2532A EPROM requires a different programming voltage than the 2532 EPROM.

Below is the 8K ROM which is used to program the MCM68764 EPROM.

ZX80 - 8K ROM (ZX81 ROM)

If you have not got a suitable programmer to program the MCM68764 EPROM , then I will be happy to provide you a programmed EPROM.

It is also possible to update an 8K ROM ZX80 with a suitable SLOW mode board, thereby giving it the equivalent functionality of the ZX81, including flicker free graphics. This board has to be soldered to the ZX80 PCB. See the Grant Searle's page of how you can build such a circuit.