Here we have a nice straightforward Reset circuit which should work correctly on power up as well as when the Reset button is depressed. Whilst the Reset button could conceivably be omitted, I generally fit one as a matter of course. Other families of logic may be used in place of the 74xx14 such as the CMOS 4000 series 40106. Watch for the correct pinouts though!

This dual one-shot is the heart of my little programmer! Although it would be perfectly feasible to incorporate a ‘delay’ into the software when the EEPROM is actually being programmed, (the programming time for each step varies from actual device type to type.) , I decided to make things easy in my design by using some simple hardware to do the job instead. This crafty little idea here switches in the ‘delay’ via one of the 374 outport lines which slows the processor right down by using the ‘Wait’ line. The /M1 Z80 line then activates the delay at the beginning of each step.

Using the industry standard charge-pump single supply rail Maxim device, the MAX232, we can dispense with any additional + and - 12Volt supplies. There are many other more up-to-date devices on the market (if one happens to be more easily available) some of which don’t even require the charge-pump capacitors. Ensure all caps are rated to a minimum of  16 volts!

Button interfaces can vary from the most simple without ‘debounce’ (like this one), to more complex variants with changeover switches and cross-coupled Nands. Due to the way the EEprom programmers program was written, this one works just fine. The 74125 interface to the Data Bus can be substituted by any other suitable tristate device, e.g. the 74xx244/241/374 etc.