Issue 2 ????

[Prime]

I have both Issue 1 and Issue 3 boarded ZX81s, though Issue 1 seem the most common to me.

Was there ever an Issue 2 ? if so does anyone have a picture of it, I'm just trying to ensure clearence for the ULA replacement.

Cheers.

Phill.

[Andy Rea]

can't say i ever saw an issue 2 board ...

[Moggy]

I agree with Andy, in thirty years of buying and selling I have never seen issue 2 boards.
Though, oddly enough, I have 3 issues of ULA's.

[zx81jens]

hio moogy!

can you tell me (us - the unaware) about these 3 ULA´s ?! please... i only know two of them!

greetings
jens

[Moggy]

hio moogy!

can you tell me (us - the unaware) about these 3 ULA´s ?! please... i only know two of them!

greetings
jens

Sure Jens there are the two common ones 2C184E and 2C210E.
Then there is the earlier 2C158E 05 ( the ones I have are date marked 81 13)
They are usually found on boards that have the rom with the SQR problem or the ones with the circuit board on top of the Z80 that corrected the SQR problem.
Very rare but they do exist, in nearly thirty years of collecting I have only come across 5 of them.

Regards Moggy

[zx81jens]

thank you moogy!

i never saw one of these...

greetings
jens

[Moggy]

thank you moogy!

i never saw one of these...

greetings
jens

Here is the ULA and the modified Z80 from the same board.

Regards Moggy

[Andy Rea]

Cool, i never saw one of those either, is there a diagram of tha little circuit around, so we can see what it's doing ?

Andy

[Moggy]

Cool, i never saw one of those either, is there a diagram of tha little circuit around, so we can see what it's doing ?

Andy

Hi Andy

The maths cockup, it seems, was caused by three bytes at 1733H-1735H and were simply deleted in the improved ROM

I have no diagram circuit for this but Ian Logan says in His book; "Understanding Your ZX81 Rom"

" Although it is as yet unconfirmed by Sinclair, it would appear that the hardware add-on that corrects the arithmatic on some of the 'old' 8K ROMS works by nullifying the effect of the 'three bytes'.
Probably when the location 1735H is addressed the instruction fetched is 'LD H,A' but the instruction passed to the Z80 is 'DAA' "

This seems to imply that it was a Sinclair fix, yet not too long ago a Guy was selling a Zeddy with this mod on fleabay and He said in the description that it was an aftermarket solution by an outside firm,(Unamed), personaly I think not on cost grounds alone,
( Uncle Clive was notoriously tightfisted), plus the Zeddies I took apart with this mod had the original factory type glue under the feet and did not look to have been taken apart previously; but I may be wrong


Hope this helps

Regards Moggy.