zx81 to zx80 conversion?

[lusciousthelock]

I read somewhere, not sure where but I read that its possible to convert a ZX81 into a ZX80? I guess a 4k rom would be needed. Is this possible and if so how? Also is a copy of the 4k rom available for sale anywhere?

[gozzo]

Well, you can turn a zx80 into a sort-of 'semi' zx81 by fitting the zx81 ROM in, but it lacks the zx81's NMI 'SLOW' circuitry... doing the opposite DIDN'T work when I tried it a while back, most probably because of this difference as triggering NMI's with a zx80 ROM in a zx81 appears to causes a crash..maybe the NMI generator activates itself on machine power-up. Could try disconnecting that transistor connected to the NMI and HALT lines... I haven't yet done so....maybe if I get time..!

[gozzo]

As for the 4K ROM being on sale, original ones would probably fetch a fortune, if any ever turn up , but the ROM file is freely around on the 'net so you can burn it into an EPROM of your choice (ideally a 2532 which although turns up fairly regularly on ebay, is unlikely to be programmable with modern programmers as they usually need 25 or 21 v program voltage, other types can be used but have different pinouts and you will need an adaptor)

[RWAP]

You can always just program an EL-2364 Flash EPROM and use that ....

[lusciousthelock]

Yes I'm sure you had to add some resistors to halt the NMI so it would work otherwise it would crash. I just cant remember where I read it? To be honest though, I know absoloutly nothing about eproms etc. I'm more of a buy it and solder it in kinda guy. I could find the files, but wouldnt have a clue what to do with them to make a chip. I was just wondering whether this was a common conversion since the zx81 is so cheap and zx80's so expensive? Obviously the 81' is the better machine but I quite enjoy finding ways round the things it lacks and been having lots of fun with the emulator.

Roll on the 28th! Payday, and I'll see a nice ZX80 kit heading my way

[siggi]

Beside using a ZX80 rom image in the ZX81 hardware a small hardware modification is necessary to disable the NMI-generator of the ZX81-hardware at startup to avoid NMIs, which cannot be handled by the ZX80 rom and would lead to a crash.

Details can be read in Tom's forum (use e. g. google translator if you don't understand German):

http://forum.tlienhard.com/phpBB3/viewt ... t=NMI+zx80

HTH
Siggi

[1024MAK]

Google translated with additional rewording my me:-

A ZX81 consists essentially of a ZX80 with additional NMI logic for the SLOW mode. Because of the great similarity in the hardware then you could upgrade a ZX80 with an 81 ROM.

Nowadays, a ZX81 is much more common than the rare ZX80. The reverse of the "ROM downgrade" from the ZX81 to ZX80 may work but not very well. A small additional circuit needs to be added to make it work reliably.

The starting point was a ZX81 board with socketed ROM for me. First, the difficulty that standard EPROMs are not pin-compatible with the ZX81 ROM, and so had to be tinkered with by making a small adapter from several IC sockets. Since the used standard-ROM offers a whopping 64 Kbyte space of type 27C512, I added a DIL switch (piano style) for the switching of 8K pages. So I can easily switch between ZX80 and ZX81 ROM. Instructions for this part can be found in www.

The problem is that the NMI generator in the ZX81 ULA IC after the machine starts is in an undefined state. If the NMI mode is active, the ZX80 ROM does not work. Only when the NMI mode was accidentally turned off when starting, the Zeddy runs as a ZX80. In my machine that was very rare, in about 9 of 10 attempts it did not work.

Therefore, following Sinclair typically extremely cheap trick with four resistors:

Code: Select all

    Z80CPU

    /IORQ  ------[R2K2]------ GND

    /WR    ------[R2K2]------ GND

    /A1    ------[R2K2]------ GND

    /A0    ------[R2K2]------ +5V


       (All resistors 2.2K Ohm)

The ROM has been replaced in the system by a 27C512 with page selection DIP switches. On the CPU you can see the 2.2k ohm resistors.
ZX81_Board_TO_ZX80.jpg (134.93 KiB)

So how does this work? The first command in the ZX81 ROM is OUT (FD),A and this switches off the NMI mode. The resistors do exactly the same during the reset phase of the Z80 CPU at power up and turn off the NMI mode in the ULA IC. It makes use of fact that the Z80 CPU puts its bus signals to high impedance state during reset, and so the resistors can take control and issue the OUT command. After the reset, the CPU permanently "overrules" the resistors, thus not disturbing the normal operation.

My Zeddy now starts reliably either as a ZX80 or ZX81.

Current ZX80 software on the "ZX81 -"
ZX81_to_ZX80_running.jpg (129.38 KiB)

[MOB-i-L]

I found the following at http://www.zx81.de/english/zx80_e.htm:

Take the zx80.rom file and burn it into a 2532 EPROM (not 2732!). Replace the ZX81 ROM with this 2532 and disable the NMI signal from ZX81 ULA by cutting the connection from ULA to Z80A CPU.

Maybe one could hack the ZX80 ROM for the ZX81 before burning it to the EPROM so that it shuts off the NMI-generator at boot, so one doesn't need to modify the hardware. Then one could also modify the "keyboard driver" so that the ZX81 keyboard works with the ZX80 ROM for ZX81.