ZX81 Enhanced ULA replacement module.

[RWAP]

There are now some more ZX-ULA2 modules up for sale on sellmyretro.com

Having discussed this in detail with Andy, we are selling them on his behalf, and he will fit them into the ZX81s.

More details appear at: http://www.sellmyretro.com/offer/detail ... ED%29-1984

[1024MAK]

Hi all

Here are three pictures of the Zeddy that I fitted with the ULA replacement module.
I was using two "dead" ZX81's and working out the causes of the failures on each. Both had been "played with" by someone before I got my hands on them, as some of the pins on the ROM's and one of the ULA's were bent. Both now work. In the end, the faults were due to one defective ULA, an incorrectly fitted ROM and a faulty modulator.

ZX81 with Andy's replacement ULA.jpg (51.62 KiB) Viewed 13108 times

ZX81 with Andy's replacement ULA - close up view.jpg (46.7 KiB) Viewed 13108 times

ZX81 with Andy's replacement ULA - displayed image.jpg (10.64 KiB) Viewed 13108 times

A big thank you to Andy Rea for the ULA replacement module

Mark

[sirmorris]

That program'll never work. just saying.

[Andy Rea]

<PEDANTIC>
hmmm GYBHVVHHBYYGGV55666VBBBBBBBBHVV6JJJN looks like a valid sinclair variable name to me....

so line 1 might be

1 LET GYBHVVHHBYYGGV55666VBBBBBBBBHVV6JJJN = 0

</PEDANTIC>

andy

[iturbez]

Good job!
Congratulations!

[sirmorris]

@andy :¬P

[Andy Rea]

Hot off the press...

New code developed for ULA module, Allows turbo mode to be used with the slow roms... the problem with the slow roms is the M1 fetch cycles is too short to allow reliable aquasition of the data, so by adding some small logic (2 flip flops) an extra wait state can be introduced ONLY when Turbo is active AND ONLY for M1 fetch cycle to the rom, code in ram will continue to execute without additional wait states.

used in conjunction with Wilf Rigter's why wait circuit ( yeilds approx 9% speed increase ) we can acchive a CLCKFREQ score of :-

frames taken : 914
speed: 203.8 %
effective clk freq 1.63 Mhz

Which is only some 24% behind a zeddy with the faster type rom.

Of course speed is unaffected when Turbo is NOT active.

to use this, you need 1 additional wire, between ULA and zx81 mainboard, connnects to the wait circuit, and optionally you can also include the whywait.

I'd be happy to do the modification free of charge, but you must pay postage both ways.

Regards Andy

[DerekJC]

Are these replacement ULAs still available or at least the technical specs/firmware?

Derek

[dessony]

Andy Rea,

When will you repost this instruction? I tried it but I got is only "can't display" screen.
http://www.debraclayton.webspace.virgin ... ulamk2.pdf

I bet that the TS1000s' / ZX81s' original ULAs are getting very rare as time flies by! One of ULAs blew up, no long working.

When will you do something about this situation?

Thank you very much in advance.
Dessony

[mrtinb]

When will you repost this instruction? I tried it but I got is only "can't display" screen.
http://www.debraclayton.webspace.virgin ... ulamk2.pdf

Internet Archine WayBack Machine is your friend. http://web.archive.org/web/201209062350 ... ulamk2.pdf