ZX97 Lite

In 1997 Mr Wilf Righter designed ZX97 - a ZX81 equivalent computer without dedicated ULA chip, with all logic made using 74HC-series chips. ZX97 contained additional RAM disk and parallel port. It was quite large, as it was assembled on 3 Euro-card format modules.
In 1998 a lite version has been designed, which could be fit on a single-sided PCB (yet with lots of wires). This "lite" version has no parallel port or RAM disk, but still has tape I/O.
This is my unit, built from original ZX97Lite PCB, using 74HC logic.

Manufacturer	-
Origin	-
Year of unit	2014
Year of introduction	1997
Type	Computer
CPU: Z80 RAM: 32kB ROM: 128kB Tape I/O, serial I/O in tape port, monochrome video. Compatible with ZX81.

To build your own unit, you need the following information:

- HERE is a PCB tracks drawing (with one mistake). Schematic has one more diffeence - Pin 4 of U10 is connected directly to /RFSH on PCB. If image doesn't download - change slash to proper.
- HERE is a schematic, parts list and keyboard description. There's also a ROM dump.
- Here's a copy of it.
- Here's a Croatian page of someone who also built a ZX97Lite. More of his photos: here

This placement sheet will help you to figure out what goes where. Remember to double-check for shorted tracks. During assembling IC sockets may require machining to fit over wires.

There are following errors on PCB:
- RAM /WR Line (pin 27) should have pull-up resistor
- U14 pin 14 should be connected to Vcc, NOT GND!
Here is another version of PCB which has fixed the last error.

ZX97Lite can be emulated by EightyOne emulator.

Here are keyboard stickers for ZX97Lite. I've used keys from old programmable keyboard so I could remove small piece of transparent plastic and insert them under it.

Keyboard layout (from EightyOne emulator):

The difference between this and ZX81 are 3 keys with commands DIRECTORY (DIR), HLOAD and HSAVE.
- DIRECTORY displays RAM disk directory. Because there's no RAM disk in ZX97Lite, rOM disk with assembler/disassembler is displayed.
- HLOAD loads program from parallel port
- HSAVE outputs current program to parallel port. Because ZX97Lite has no parallel port, these 2 commands just hang ZX97lite.

Example usage of ROM disk: Loading assembler:
- DIRECTORY [return]
- With 5/6/7/8 point to ASEM8192.
- L to load assembler
- Q to return to BASIC.
- RAND USR 8192 - runs assembler.

One more thing: While testing my unit, I've found some problems with U20 and U21. The symptom is when inverted-video character is displayed and some lines of it blink. I've fixed this problem by adding 100n decoupling capacitor to U21.