Sinclair ZX80 / ZX81 / Z88 Forums

I've not seen any mention of this here so I thought I'd share a link.


http://blog.tynemouthsoftware.co.uk/201 ... utput.html

HackaDay had a blog about this last week http://hackaday.com/2017/01/05/a-thorou ... lair-zx80/. It is obvious the author of this blog doesn't get around to the Sinclair forums since she makes it sound like the Minstrel is the only modern clone but we know better

Hi all,

I'm curious if anyone here has built one of these yet? I was thinking about ordering a pcb to test it out.

Best,

-mark

I bought a pre assembled version and its nice and well made. I found some graphical glitches when using it in ZX80 8k rom mode, but seems fine in 4k rom mode. Not sure what the issue is, but every other keypress makes the character that the curser sits on look corrupt? Anyway, made a tidy perspex case for mine and added pokemons keyboard.

And it fits inside a Zeddy case. The mic, ear, composite video line up. Issue 2 has the bus edge connection.

I've just purchased an issue 2. I can't seem to get it to load anything just yet but that might be because I don't have the volume on my tape quite right yet either.

As it has 4K and 8K ROM built in, does that them that if I can add in the right hack, it'll essentially be a ZX81 in 8K ROM mode?

Also it has a 6.5 Mhz quartz in it. I assume half of this is what it's processing at, even though the Z80 CPU is 6Mhz and therefore being under-clocked.

Am I correct in assuming that replacing the quartz with a 12Mhz one may allow the CPU to run at 6Mhz?

Regards,

Shaun.

if you do that, you wont be able to get a display as the video timings are derived from the clock, and yes, the cpu clock is half that of the resonator/crystal...you must keep it at 6.5mhz...although it IS possible to rig up a 'turbo' circuit, which runs the cpu at 6.5 mhz when running program code, and the critical 3.25mhz on display code... details are around on the net somewhere..

a 6mhz cpu was probably used as it may have been more easily available than a 4mhz one.....

Both the ZX80 and ZX81 use a 6.5MHz resonator. In the case of the ZX81, it is the ULA that divides the 6.5MHz by two to give 3.25MHz, which is then fed to the Z80 CPU.

Sinclair used the 4MHz NMOS version of the Z80 (or licensed Z80s, or unlicensed Z80 clones from NEC).

Now, all new Z80 CPUs are CMOS and the 6MHz version is far more common than the 4MHz version.

A ZX80 will run a ZX81 8k byte ROM. But it can only run in 'FAST' mode, as the ZX80 lacks the circuitry to support compute and display 'SLOW' mode. I presume the Minstrel ZX80 is the same.

Mark

I've built one. Everything works, except. Igot this black band on the left, as though extra screen. I'm using a 6.55 mhz crystal. May be its that.

Thanks. Any ideas

David

msellan wrote: ↑Thu Apr 13, 2017 3:11 am Hi all,

I'm curious if anyone here has built one of these yet? I was thinking about ordering a pcb to test it out.

Best,

-mark