excellent video by JoulesperCoulomb on a solution to a similar problem on the ZX81, which explains the requirements in more detail.
Grant Searle's redrawn version), the video signal is a signal with either 0V for black and 5V for white, and sync is 0V when a sync pulse is required, 5V normally. (N.B. these are TTL, so it's not quite 0V or 5V, but good enough for the purpose). These two TTL level signals were merged together with two resistors and fed to the UHF modulator.
Paul Farrow's website.
- Fixes from Issue 1 - A6-INT link and keyboard connector pinout now corrected
- Edge connector - several people have asked for this, so I have added an edge connector, complete with the ROM_CS line which was not present on the ZX80.
- Reset button - I've found this useful, so have added one, can be wired to an external switch if you prefer.
- Integral functions list - Several keywords on the ZX80 (such as PEEK which confused me last time) need to be typed in separately. There is a list on the ZX80 case, so I have added one to the PCB silkscreen.
- ROM selector jumpers - There is now the option to select form up to 8 ROM images, so you can have ZX80, ZX81, Forth, Assembler etc. if you choose.
- Regulator position - I've shuffled the power input around so it can be bolted to the board and to any random bit of metal such as a ZX81 heatsink.
new blog post for more info and buy it now links.
I have found some TVs work better with a lower video level, so I have adjusted the values of the two summing resistors to reduce the signal level. Current revisions of the board and kit include these new values.
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