An upgraded / replacement ULA for the Sinclair ZX81 is now available.
The ZX-ULA2 is a direct replacement for the standard ULA used in the Sinclair ZX81 home computer.
The ULA is a custom chip designed by Sinclair Research Limited and is responsible for control of the functions of the ZX81, including the screen. Early ULAs were dogged by the lack of a back porch signal, which results in black text on a dark background on modern colour and LCD TVs. Until now, if you had one of these chips, you were stuck with either looking for one of the later ULAs (which are rare), or installing a modification, such as the ZXvid module, which alters the composite video signal and adds a back porch signal.
However, the ZX-ULA2 has now been designed to replace both of these ULAs and provide extra functionality for the Sinclair ZX81 at the same time. It will also work to turn a ZX81 in ZX80 mode (with a suitable ZX80 ROM), and also works with the Timex-Sinclair TS1000 computers (but not the TS1500).
Features include: -
• Built in composite video circuit, complete with back porch signal
• Switch between 50Hz and 60Hz output for testing of software which may be used in the USA and Canada
• Switch between black no white or white on black screen output
• Enable the M1Not modification - this allows machine code routines to run in the 32K-48K RAM region of memory.
• Turbo function, which enables the machine to operate at 6.5Mhz compared to the original ZX81 speed of 3.25Mhz. In SLOW mode, the computer will automatically drop out of TURBO to generate the display and switch back when performing another operation.
Each feature can be controlled from within software using a simple POKE command, thereby allowing you to disable Turbo for some programs, and enable it for others.
You can purchase the ZX-ULA2 from SellMyRetro.com