ZX81 strange keyboard problem?

Anything Sinclair ZX Basic related; history, development, tips - differences between BASIC on the ZX80 and ZX81
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1024MAK
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Re: ZX81 strange keyboard problem?

Post by 1024MAK » Thu Jul 20, 2017 9:59 am

hyttju wrote:
Thu Jul 20, 2017 9:38 am
I can not change line so it is impossible to run commands.


Here is a picture of what came up when i wrote

10 print "hello world"
And what happened when you pressed the [NEW LINE] (enter/return) key?

Mark

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sirmorris
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Re: ZX81 strange keyboard problem?

Post by sirmorris » Thu Jul 20, 2017 10:12 am

Something is wrong with the addressing logic used to select the character data. The ULA uses the character code in the dfile as part of the calculated address of the character's bitmap data in ROM. It's consistently wrong, not random.

The ULA looks in the d-file and sees '1' but produces '-'. 'P' is unaffected. As are L,O,R and space.

Code: Select all

'1' = 1Dh,  00011101b

'-' = 16h,  00010110b

'P' = 35h,  00110101b
So to turn '1' into '-' you remove bit 3. Bit 3 is clear in 'P' and therefore it remains unaffected.

Because the computer is working this problem with A3 has to be internal to the ULA. If A3 were damaged on the processor side then seemingly random opcodes would be executed and the computer would not work.
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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Andy Rea
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Re: ZX81 strange keyboard problem?

Post by Andy Rea » Thu Jul 20, 2017 10:46 am

sirmorris wrote:
Thu Jul 20, 2017 10:12 am
.

Because the computer is working this problem with A3 has to be internal to the ULA.
Or it is not connected / dry jointed at the ula

We gotta be optimistic that the ula ain't dead 😁

Regards
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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Andy Rea
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Re: ZX81 strange keyboard problem?

Post by Andy Rea » Thu Jul 20, 2017 11:02 am

I'm.more convinced on the open circuit idea now, i. The word PRINT the wrong letters à3 is high but in the word HELLO the wrong letters a3 is low this pattern will reapeat every 8 characters as à3 is following the refresh register as it counts down across the screen...

Regards
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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sirmorris
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Re: ZX81 strange keyboard problem?

Post by sirmorris » Thu Jul 20, 2017 11:34 am

Andy Rea wrote:
Thu Jul 20, 2017 10:46 am
Or it is not connected / dry jointed at the ula
Yes that's a possibility but then other functions that rely on a3 would be broken - kb input for example.

I didn't carry on to check my theorem so I missed the fact that the other wrong letters had a3 pulled high. Shame on me :lol:
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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1024MAK
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Re: ZX81 strange keyboard problem?

Post by 1024MAK » Thu Jul 20, 2017 11:40 am

I think an internal ULA fault is the most likely. However, a poor connection could result in some functions working, while others don't work. As the impedance of the electrical circuit may affect the voltage level.

Mark

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sirmorris
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Re: ZX81 strange keyboard problem?

Post by sirmorris » Thu Jul 20, 2017 11:54 am

Code: Select all

E A   E  A   E     A
- -   -  -   -     -
1 -   1d 16  1101  0110
0 =   1c 14  1100  0100
p p   35 35        0101
r r   37 37        0111
i a   2e 26  1110  0110
n n   33 33        0011
t l   39 31  1001  0001
h 9   2d 25  1101  0101
e 6   2a 22  1010  0010
l l   31 31        0001
o o   34 34        0100
w o   3c 34  1100  0100
d 5   29 21  1001  0001
Bit 3 forced to zero in every case. Well that killed 10 minutes :?
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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sirmorris
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Re: ZX81 strange keyboard problem?

Post by sirmorris » Thu Jul 20, 2017 11:55 am

sirmorris wrote:
Thu Jul 20, 2017 11:34 am
I didn't carry on to check my theorem so I missed the fact that the other wrong letters had a3 pulled high. Shame on me :lol:
Double shame for doubting myself :oops: :lol: :lol:
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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Andy Rea
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Re: ZX81 strange keyboard problem?

Post by Andy Rea » Thu Jul 20, 2017 12:12 pm

Oops sorry everyone think my brain is addled :oops: Guess this is what happens when one neglects the humble zeddy for so long
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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Andy Rea
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Re: ZX81 strange keyboard problem?

Post by Andy Rea » Thu Jul 20, 2017 12:31 pm

Oh one more thing, just to be sure I'm completely bonkers and haven't a clue as to what's going on... we all going bit 3 of the character code translates into bit 6 ( A6' on the ula ) because we have the line counter at A0' A1' A2'.

It is very odd that it's only presenting a logo. Low during character generation and not all the time. But i guess it is looking like the ula is frazzled.

Regards.

P.s. do they do certificates in crazy :mrgreen:
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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