ZX80NMIX Boards

[PokeMon]

So the board is ready for sale now.
But there have been 3 mistakes on the board which can be repaired.
Two of them are easy, the last one is a little bit hard but can be made with some experience in soldering.
So I will offer this item not as soldering kit just as "soldered, adjusted and tested".

By the way, there is one more change, C6 is changed from 470pF to 1nF.

1. Mistake
There was one missing connection, I will do this with a white wire but smaller than in the picture below (half diameter than used in the picture). This was caused through the layout program DesignSpark. In schematics there can be place wires over each other even with a knot (connection) but can be 2 different nets which look like one net. Well - a pity but could happen.

2. Mistake
This can be repaired simply as well. Pin 4 of IC1 (74HCT40103) has to be connected to GND instead of VCC. This can be done simply by lifting the leg of #4 and put a small wire to #3 (which is connected to GND). I made this change on my prototype board but unfortunately forgot to mark this change. So I missed it - can happen, too.

3. Mistake
This is the trickiest one and I thought I could replace a 74LVC1G126 driver gate with a 1/4 OR gate (74HCT32). This is the WAIT logic which add wait states AFTER the HALT (when HALT goes high as long as NMI is low). So I simplified it in my mind with adding WAIT states when HALT is low - which is wrong and results in a bad first line in the picture. So I have to add an additional driver IC on the back side of the board and add one wire. And another wire has to be cutted, the driver placed over both track parts over a 6 mil (0.15mm) wide track where soldering lack has to be removed. I would use a small 74LVC2G14 in DCU package with 0.65mm pitch. This is not easy to prepare and solder and so I decided to do it self. Well this is a design error and should not happen but in life everything can happen.

All in all the board looks not too bad. So please write me a PN when you want to have this board. It will be listed on sellmyretro.com soon (when proofed with some other customers). The picture quality is good and you can switch between SLOW and FAST with the ZX81 ROM. Of course this board works as well with ZX80 ROM as the NMI generator is disabled after powerup.

I have to test but I think the FORTH rom burned into the bigger 28C256 will work as well with this NMI board.

[PokeMon]

Board front:

ZX80NMIX_board1.jpg (220.02 KiB) Viewed 3308 times

(Only the second wire at the bottom is used. The upper wire caused through testing/removing the board track manually).

Board backside (not visible when inserted in ZX80CORE):

ZX80NMIX_board2.jpg (48.56 KiB) Viewed 3308 times

(Here you see the manual added inverter for the /HALT signal)

The new schematic:

ZX80NMIX - Project.pdf

(50.28 KiB) Downloaded 173 times

[PokeMon]

Screenshot of normal CRT television, ZX80CORE with ZX81 rom in SLOW mode:

ZX80NMIX_SCR3.jpg (204.13 KiB) Viewed 3308 times

Screenshot on modern Plasma TV with better resolution (1024x1024 pixel):

ZX80NMIX_SCR1.jpg (218.8 KiB) Viewed 3308 times

Zoomed clip of Plasma screen:

ZX80NMIX_SCR2.jpg (202.68 KiB) Viewed 3308 times

[PokeMon]

There are 2 trimmers on the board.
The upper one (R5) is for match the exact starting point of the first visible line.
The lower one (R6) is for match of the correct scanline (reset of counter).
The best way to adjust is to write a small test program which prints the screen full of best inverted characters or at least mixed inverted and non-inverted chars.

The trimmer R5 adjust only the very first line and does not has effect when only showing the last line. So a listing will do as well.
The correct scanline can be found at best when showing inverted chars in the first line. So there should be one black pixelline on top and one black pixelline at the bottom of the character so placing the character in the vertical middle. Be sure to adjust the scanline in "NEWLINE" mode, when ZX81 rom is accepting any input/command like LIST or EDIT because the scanline counter is reset through keyboard check (keypress checking) when a program is still running. In this case you won't see any influence of R6.

I checked the weight of the board (about 7 grams).
So when you can solder the connector to the ZX80CORE (2x8 pin) yourself I could offer a shipment for just GBP 1.00.
Because the height of the letter have to be maximum 10mm (0.4 inch) this shipment could be done only with connector separate (not soldered).
Of course it is included in delivery, you just have to solder it in the board.

Otherwise shipment would be GBP 3.00 when height increases above 10mm (0.4 inches).
So write me a PN and let me know how you like it.

[RWAP]

Pokemon (Karl) - for the uninitiated, could you explain the point of this board and how it will fit...

I presume it will work with the original ZX80 (and clones) as well as the ZX80Core.

[PokeMon]

Well, Rich,
I am not sure if splitting the threads is more helpful but there maybe reasons for doing as for avoiding as well.
The ZX80-NMIX board is an addon for the ZX80CORE board to support the SLOW mode when using the ZX81 rom in the ZX80 hardware.
This board will fit directly into the connector of the ZX80CORE (J96).

The ZX81 rom will run either in FAST mode without this ZX80NMIX but there are other ROM / OS which need this NMI and are running otherwise only in the original ZX81 hardware (the Forth "H4TH.rom" for example).

The ZX80CORE thread can be found here:
viewtopic.php?f=6&t=1134


About the other question, it may be used with an original ZX80 as well as most of the signals are available at the board.
There is just the scanline counter which is discoupled via a resistor 2k2 on the ZX80CORE. This resistor is not existing on the original ZX80.
I think this is the only modification necessary, disconnect the connection from IC11, pin 11 (74LS00) to IC21, pin 2,3 and put a resistor between and of course connect 16 signals manually to the board. I am not sure if this would be from interest.

It is a bit different to the solution of Grant Searle.

[RWAP]

I agree that splitting the thread may not be the best approach - however, I was thinking about someone new coming to the forums, or maybe someone who has not read the whole of the ZX80Core thread.

Those who may be interested in purchasing the ZX80Core will want to read about that motherboard and this is then lost by the last 2 pages of the thread talking about the ZX80NMIX - I will add another comment to the end of the ZX80Core thread to clarify based on your above comment (as I was not sure what this board did!).

[PokeMon]

So today I finished the first 7 boards and will ship tomorrow in order of the payments.

I added some new pictures with the smaller white cable on top and how it looks like in a ZX80CORE.
If necessary you could recalibrate in your own ZX80CORE a litte bit - I described above how to do and what to look for.
Have fun.

The Forth rom H4TH.rom doesn't work with the ZX80NMIX - don't know why exactly. It should run in a original ZX81 and it does work in the emulator EightyOne if you configure enough RAM and "protect ROM from writes". So the EEPROM was protected against writes in my ZX80CORE due to the hexcode switch settings. Maybe the ROM needs the NMI generator active by default - my board is passive by default and has to be switched on. If I have time I can look at the code but maybe someone of you know why it doesn't work or has experience with this ROM in a ZX81.

Here are the latest pics:

IMG_7024k.JPG (164.97 KiB) Viewed 3213 times

IMG_7025k.JPG (125.82 KiB) Viewed 3213 times

IMG_7200k.JPG (205.99 KiB) Viewed 3213 times

[PokeMon]

Some minor changes in schematic:

(C5 and R4)

ZX80NMIX - Project.pdf

(50.32 KiB) Downloaded 206 times

[stefano]

So cute