Sinclair ZX81 Edition 1 ROM Hardware Fix

The ZX81 was released with two editions of its ROM. The first edition had an infamous bug that resulted in the SQR 0.25 yielding a result of 1.3591409 instead of 0.5. The bug was caused by 3 unnecessary instructions that introduced corruption into the computed result. Rather than throw away all ROMs that had been manufactured, Sinclair crafted a small hardware board that was soldered across the Z80 CPU. It intercepted the execution of one of the erroneous instructions and changed it such that the instructions no longer had an effect on the floating point calculation. Due to the manner in which the hardware board intercepts the instruction, it will also intercept and override an instruction fetch at the corresponding location within a ROM cartridge. This places constraints on the programs that can be run from a ROM cartridge when plugged into a ZX81 fitted with the hardware fix board.

Sinclair ZX 16K RAM Pack

The Sinclair 16K RAM pack is compatible with the Chroma 81 interface but requires that the onboard 16K RAM is disabled by setting configuration switch 1 to off. If the switch is set to on then it will override all memory accesses to the Sinclair 16K RAM pack. The Sinclair 16K RAM pack must be connected behind the Chroma 81 interface.

Sinclair ZX Printer

The Sinclair ZX Printer is compatible with the Chroma 81 interface. The ZX Printer must be connected behind the Chroma 81 interface.


The ZXpand SD card interface created by Charlie Robson is compatible with the Chroma 81 interface. The ZXpand contains 8K of ROM and 32K of RAM, and these will be overridden by the corresponding memory of the Chroma 81 interface if enabled. By combining the RAM from the ZXpand with that of the Chroma 81 interface, it is possible to achieve a contiguous block of RAM in the 16K-64K region.

64K RAM Packs

64K RAM packs are compatible with the Chroma 81 interface and should be connected behind Chroma. If any of Chroma's RAM control switches are set to on then the Chroma RAM will override accesses to the corresponding memory area of the 64K RAM pack.

Internal 4K Graphics ROM

Kayde Electronic Systems and dk'tronics both released a 4K Graphics ROM board that plugged internally in the ZX81 and provided 7 alternate character sets. The Chroma 81 interface will correctly display the alternate character sets if such a board is present inside the ZX81. However, the use of a ROM cartridge might prove incompatible when there is a Graphics ROM board connected in the ZX81.

NTSC Support

The Chroma 81 interface operates correctly with a 60Hz ZX81 (or TS1000).

Memotech Keyboard

The keyboard interface produced by Memotech can be connected at the same time as the Chroma 81 interface and will operate successfully. However, due to the way the keyboard interface circuitry works the joystick socket of the Chroma 81 interface must not be used while the keyboard interface is connected otherwise damage is very likely to occur to the Chroma 81 interface.

TV Compatibility

Several models of TVs by Panasonic do not appear to implement the SCART specification correctly and will either shift the picture to the left or cause the picture to continually jump left/right. It appears to be due to the TV expecting the video synchronisation signal to contain a colour burst signal, but this should only be required for an RF modulated signal.

Use on a ZX80

The Chroma 81 interface has been designed for use with the ZX81. Although the ZX80 hardware functionally appears to be a subset of the ZX81, there are some subtle differences in its operation that prevent some facilities from working correctly, e.g. colour modes. Instead the dedicated Chroma 80 interface should be used with the ZX80 (irrespective of which version ROM is fitted inside the ZX80).

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