ZX80 chips reduction

The current page is not complete! I am working on it right now.
Do not build anything on this page yet!

Further hardware development of the ZX-80 on 2019? Yes! A ZX-80 with less chips count? No way, the computer is already too simple to be simplified more, one could argue. If they knew it back then, they would have probably already done that to reduce cost even more. Or maybe the cost of the LS series logic back then was not high, so it wouldn't worth it. Or maybe computer-related people would not want to mess around with discrete circuits. Who knows! The point is, that I begin to give an answer to the disbelievers.

To reduce the chip-count of the ZX-80, an easy way I thought was to replace the simple-gates ICs with discrete logic. This may initially seem easy, but the thing is that it is not as straight forward as it seems. The purpose of replacing the IC logic with discrete, apart from fun, is to minimize cost and for availability reasons. For example you may not have the particular chip on hand but you may have a few general purpose transistors and diodes available. The key point is, that complexity has to be kept in a reasonable level in this discrete logic, so you cannot simply recreate the internal schematic of an IC using discrete components. What you can do instead to keep complexity down, is to use discrete components to mimic the behaviour of an IC gate at a great extent, so the rest of the circuits of the computer will be happy with it.

This requires a lot of testing and you have to test all the circuits together working as a whole, not just bits. The gates I built, are designed to operate correctly on this computer only, most of them cannot be used as general purpose replacements for the IC types they suppose to replace. Sometimes I made a gate and then, when I made its surrounding connected gates, I had to alter or re-think this particular gate to properly cooperate with the surrounding gates connected. A particular problem I faced, was that the gates inside an IC was used at different places in the computer and  connected with different external gates. So a particular discrete circuit replacing a specific type of gate, may be suitable in one place but not in the other without modifications. Also, note that the three gates in the main clock of the computer have been replaced with a discrete oscillator circuit, that uses the ceramic resonator on the computer board as a resonating element. The output level of this oscillator is rail to rail (0 to 5v) and it approximates a square wave, so that the computer and especially IC9 is clocked nicely through the C9/R2 integrator. So here is the schematic I have ended up with.


To be easy for you to follow the schematic and identify which circuit replaces the relevant gate, I have drawn most the discrete gates at the locations where the IC gates originally were. I have also left the original IC numbers that the particular discrete gates refer to and also the IC pin numbers that they refer to. The lower gate previously connected in parallel to pins 11 and 10 of IC 15, has been removed. Here is the full schematic of the computer, including the discrete gates.


Below is the computer in gates-reduction testing.