Sinclair ZX80 / ZX81 / Z88 Forums

Pokemon - can you put my name down for one of the zx80nmi board,please!

Here is the schematic of the NMI generator ZX80NMIX and the layout.
The board will be with white solder mask like the ZX80CORE itself and has minimal height due to small SMD components on it.
It should fit into the original ZX80 case I think.
C4,5,6 are approximate values and could be changed little bit when testing the final board which has some other logic components. I build up the first generator manually with other logic gates/families and for space reasons I changed some to LVC logic gates (low voltate cmos).

In principle this circuit works similar to the board of Grant Searle. I used less logic gates and one more counter IC. The main difference is: HSYNC and VSYNC are not splitted like in Grants solution. I overdrive the reset pin of the scanline counter which is encoupled already on the ZX80CORE board with a 2k2 resistor (signal ScReset). After powerup the NMI generator is disabled by default and will be activated first when switching it on using the usual OUT instructions/addresses. This way it doesn't conflict with the ZX80 ROM and works fine with the ZX81 ROM.

The driver is spared on the board which will work together with the ZX8-CCB on J94 near the modulator. The video trimmer of this board has to be adjusted slightly because the LVC gates are operating with 3.6 Volts only which result in a little bit lower video all-in-all level.

Both trimmers on the board are for fine tuning and adjustment. C6/R5 forms a R/C combination to extend the reset signal created from the second timer IC. The original ZX80 impulse is a bit longer than the 16 clock cycles used (18 or 19 I think). This will be compensated by the TV on the next line but the first pixel line will be moved a little bit more left which can be fine tuned with the trimmer.

The second R/C combination C4/R6 delays (or extends) the scanline counter reset impulse. Because the ZX80 circuit is problematic as it creates an additional counter impulse when switching the NMI generator off. So the scanline counter will not be zero and scramble the pixel lines. In general the HSYNC and VSYNC pulses from the original ZX80 circuit are mixed with the pulses created by the NMI generator which makes it necessary to get these pulse as much equal as possible.

The board ZX80NMIX is double sided with white stopmask and black silkprint, 28x40mm (1.1x1.6 inches) and will cost EUR 3.40 (GBP 2.90).
The full parts set costs EUR 5.00 (GBP 4.30).
So the complete set is EUR 8.40 (GBP 7.20) for self soldering.
Or GBP 9.40 soldered and tested/adjusted.

IC7 and IC9 have 0,65mm pitch (25 mil) and require a good soldering experience/equipment.
IC2,4,5,6 have 0,95mm pitch (37 mil) and can be soldered more easy.
The big IC1,3,8 are quite easy with 1,27mm (50 mil) and the other SMD parts as well easy to solder.

So you can decide yourself.
I think boards will be available in middle of january and I leave a message here when they are available.
Or you can build your own ZX80NMIX manually regarding schematics without board - up to you.

Hi PokeMon

I'm interested in the ZX80NMIX

Mark

So the complete set (EUR 8.40) for self soldering for me, please.

How much is postage to teh Czech Republic. Can I send you PayPal?

Best regards
Martin

I will announce again in january.
I think shipment will be EUR 2.00.

Ok, I will wait.

The boards did arrive - I am waiting a few missing components and will test all at the weekend.
and the backside ...

How exciting! Nice work as always Karl

Nice! I am looking forward to it.