|• Persönlich •|
• 1. Generation ZX80/81
— Projects: History, 1. Generation ZX80/81 —
ZX80 and ZX81
The cassette interface to store programs was a real nuissance and many proud owner of the ZX81 complained about it in the computer magazines of that time. The first thing was to improve this. Also, we wanted to use ZX80 and ZX81 mode alternatively.
The picture below shows the ZX80 in the foreground with the stacked ROMs that could be selected by a switch. To the left, an interface was added that connected to the unit at left in the 19 inch case in the background. It contained the audio filters I made. From that time on, the cassette interface worked with almost 100% reliability.
The ZX80 had an edge connector that accepted an add-on memory pack of 16 KB. It was a mechanical catastrophe. Depending on how hard you pushed on the keyboard, it lost contact and another half an hour of typing was lost. Because of that, typing in a long program and save it to the cassette could take half a day.
A plug-in unit was added beside the audio filters and the 16 KB memory screwed onto the board. A ribbon cable connected the ZX80 to the RAM. The power supply to the right was designed with major hardware extensions in mind: 5 V, 5 A; +12 V, 1 A; -12 V, 1 A.
Eventually, I found used keyboard PCBs from «real» computers with gold plated traces on the PCB for a price that was slightly higher of what you have to pay for a cheap new keyboard today. My colleague built a wooden case to build the board into. An additional interface had to be built for it. It worked fine. The keyboard can be seen on top of the tray in the picture above.
The first really useful program written for the enhanced ZX80/81 system was by a colleague. It was used to determine the components for a transistor amplifier.
The above is a screen dump of the result page from an example run. I did it not on the ZX81, but with an emulator on the Pentium II I had at the time.
The BASIC program has a size of 6'791 bytes and needs 7.5 KB ROM and 512 Bytes for the character decoder. Try to match this today. You can download the program together with a game and a ZX81 emulator for DOS here (emulator.zip 38 KB).
© 2004 - 2018 by Horo Wernli.