Sinclair ZX80 / ZX81 / Z88 Forums

Hi Ollie,

It may be a wee while before Lee checks back in so in the mean time I can tell you that the intention is that a run will be made when the initial bugs are ironed out. I think the foils are available but if I were you I'd wait - there are at least 2 minor (but irritating) issues which need attention.


HTH

Charlie

Thanks Charlie

I'll wait, it will give me some time to understand the circuit.
BTW, anyone has a schematic of the original ZX80 to compare

Ollie

http://home.micros.users.btopenworld.com/zx80/zx80.html
http://www.zx81.de/english/zx80_e.htm
http://tinyurl.com/2uqc9mp

@Charlie,
I like your tinyurl best
@all
well my wife is looking a horrorfilm, that gives me some time for my legacy zx80.
I replaced all IC's against 74HC, and voila, my keyboard has also gone mad
(even worse than what I have read here):
only a few rows are fully functional, the right side mostly does nothing.
The working keys differ from zx80 to zx81 rom
I rereplaced U6 against 74LS157 and most keys work again, only some shifted keys still had some errors.
Then I rereplaced U10 against 74LS244, and everything works fine again
(all keys do what you'd expect them to do).

Cheers
Paul

hi guys!

i tried that also - all LS against HC ... nothing worked! so i change one by one. the result:

U10, U14 und U16 are still LS
U17 is AC (!) - i didn´t have a HC neither a LS - that one came with the package
the rest is HC

my keyboard work´s fine - i have no problem here (never had one).

exept the moved sync when i switch on the computer is a (small) problem

greetings
jens

Hello...

Still looking forward to this... any updates as to when it will be available?

Regards.. Mike

Hi Mike3 and all,

I'm still here; just busy. "Real life" tends to get in the way of having fun.

I have my board completed, and am investigating the keyboard problem. Indeed, it has something to do with LS versus HC logic. And it's not even so nice as saying all LS works; I found one LS package that creates keyboard problems, too. I have a feeling Uncle Clive got clever, and something in the design is operating outside of worst-case margins, and so depending on parts to be "better" than specs.

One thing that would help: Does anyone have a listing of the Z80 code that is reading the keyboard, especially the shifted keys? If I knew how it is supposed to work, that would help track down what is going wrong.

Lee Hart wrote:Hi Mike3 and all,

I'm still here; just busy. "Real life" tends to get in the way of having fun.

I have my board completed, and am investigating the keyboard problem. Indeed, it has something to do with LS versus HC logic. And it's not even so nice as saying all LS works; I found one LS package that creates keyboard problems, too. I have a feeling Uncle Clive got clever, and something in the design is operating outside of worst-case margins, and so depending on parts to be "better" than specs.

One thing that would help: Does anyone have a listing of the Z80 code that is reading the keyboard, especially the shifted keys? If I knew how it is supposed to work, that would help track down what is going wrong.

This may be of use Lee.

Regards moggy

http://www.wearmouth.demon.co.uk/zx80.htm

Hi Lee,

concerning the HC versus LS logic keyboard problem:

Did you look at the pullup-resistors R13-R17? At 47K Ohm, they might probably cause trouble, depending on the stray capacity of the keyboard line:

These 47K-pullups need to rise the input voltage above the logic threshold within one I/O read cycle (about 500ns). With a typical capacity of a keyboard line of say 10-15pF, this RC time constant could become critical (too slow).

I know Uncle Clive also used 47K for the ZX80. However in the ZX81, they reduced the R's to 10K!

Why does LS work and not HC then?

HC circuits have very high input impedance. In contrast, LS circuits slightly pull their own inputs high. So, the LS in the original ZX80 helps the 47K resistors pulling up the input line timely, while HC does not. Additionally, HC has a higher threshold level (2.5 vs 1.4).

So I think it may be worth to try and replace R13-R17 by 10K Ohm resistors, especially if one is going to connect an external keyboard.

Greetings
-oliver

What of the MicroAce, can we compair their ZX80 implementation to Lee's?