ZX80 Core - new ZX80 motherboards

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
poglad
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Re: ZX80 Core - new ZX80 motherboards

Post by poglad » Fri May 16, 2014 9:52 am

Oh sorry, the voltage (logic level) - I was rapidly falling asleep. I'll measure it tonight. I've got a horrible feeling, with the demise of the Aszmic image, that my investigations have destroyed a chip. It was good to see that flashing cursor though, and every keypress worked. Sigh.

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1024MAK
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Re: ZX80 Core - new ZX80 motherboards

Post by 1024MAK » Fri May 16, 2014 1:35 pm

Or it may just be a poor connection somewhere...

Once chips are fitted to boards and are "in circuit", their internal static protection diodes are more effective. And unless you have some nasty solder splashes or other short circuits, with just one supply (the +5V) line fed from a voltage regulator, it's hard to kill a chip.

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poglad
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Re: ZX80 Core - new ZX80 motherboards

Post by poglad » Fri May 16, 2014 6:11 pm

That's great to know. But if there were connection problems between the ROM selector and the EEPROM, couldn't that have done something nasty to that chip?

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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Fri May 16, 2014 6:47 pm

By the way, you have to reset after turning the ROM switch or turn it while power-off - I hope this is clear. ;)

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1024MAK
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Re: ZX80 Core - new ZX80 motherboards

Post by 1024MAK » Fri May 16, 2014 7:10 pm

poglad wrote:That's great to know. But if there were connection problems between the ROM selector and the EEPROM, couldn't that have done something nasty to that chip?
If you have an unconnected pin and that pin is an "input" to a CMOS chip (eg 74HCxxx or 74HCTxxx types, SRAM chips, EEPROMs, a Z80 CPU that has a "C" in the middle of the type number etc), that pin will float to a voltage somewhere between 0V and +5V. Often it floats in between valid logic levels which results in the input stage drawing more power than it should. This is NOT good. The chip will often not work as intended. If it is just one input, normally the chip will not be damaged. If however the chip is an older type, the behaviour is often more predictable. For example TTL (74xxx) and LS TTL (74LSxxx) types have inputs that normally float to a logic high.

It is rare that an unconnected input will cause damage to a chip.

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poglad
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Re: ZX80 Core - new ZX80 motherboards

Post by poglad » Fri May 16, 2014 8:00 pm

This is all encouraging, thanks guys!
PokeMon wrote:By the way, you have to reset after turning the ROM switch or turn it while power-off - I hope this is clear. ;)
Yep, I got that. :)

Okay, pin 1 - A14 - is 0 as expected. So that's something. In fact, A14 to A9 are all zero. /WE is 1 again as expected, /CE is 1 and /OE is 1. I'm not sure whether those are right or not.

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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Fri May 16, 2014 10:08 pm

How did you set J100 ?
To left or right position ?
A14 (pin 27 IC 2) should not be zero when using an EEPROM - this is for EPROM type only but will be overridden through ROM switch.

It should be connected to /WR signal which should be more to high than to low (zero).
Of course it is pending between low and high but the CPU should more read than write.
Low indicates write which causes problems to the EEPROM (non-destructable but wouldn't work in that configuration).

Depending on the switch - A14 (pin 27) should be high in any valid position (4,6,C,E).
A13 (pin 26) should be high in position 6 and E (ZX81 and ASZMIC).
A15 (pin 1) should be high in position E only (ASZMIC).

And please check the soldering points around that switch and for the EEPROM for any bad connections (cold solderings).

poglad
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Re: ZX80 Core - new ZX80 motherboards

Post by poglad » Fri May 16, 2014 10:44 pm

I have J100 positioned over the rightmost 2 pins, with one unconnected pin immediately to the left.
Okay, just to check - my IC2 is a 28C256 chip, and the pinouts I see on the internet say that A14 is pin 1 not pin 27. That would be 27C256. I maybe gave you some bad information earlier.

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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Sat May 17, 2014 12:14 am

Yes that is because I looked into schematics. There is a small difference between EPROM (27C512) and EEPROM (28C256).
So A14 in my schematics is /WR for the EEPROM (which the EPROM doesn't really have, there is a PGM pin which needs also higher voltage) or A14 for 27C512.
A15 is A14 for EEPROM 28C256 or A15 for EPROM 27C512.
So I meant to measure the pin 27 of EPROM (/WR).
What voltage do you have here ?

Please refer to page 23 (Step 9) of the manual for details or Appendix A2 for corresponding ROM address table.
So I will send you a new and fresh 28C64 with ZX81 image with your resonator.
The point is, if you got one time a valid screen for the ASZMIC rom there is some kind of instability in your system - I have no idea by now what it is.
I have some more test cases but they would need an oscilloscope.
Another approach is that I have a german candidate with some problems in his ZX80CORE which is near to Munich where I live.
So I could find the problem here at home and if I solved we can check if it is a similar problem to your board. ;)

XorA
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Re: ZX80 Core - new ZX80 motherboards

Post by XorA » Sat May 17, 2014 8:13 pm

Built my zx80core today \o/

Unfortunately it is a bit too goth for me in that in only outputs black on black :-(

My A15 seems to agree with the debugging guide.
ZX80-Core-A15.jpg
(174.9 KiB) Downloaded 162 times
I do not currently have access to my Oscilloscope only USB logic analyser :-(

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