ZX80 Core - new ZX80 motherboards

Any discussions related to the creation of new hardware or software for the ZX80 or ZX81
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1024MAK
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Re: ZX80 Core - new ZX80 motherboards

Post by 1024MAK » Sun May 18, 2014 8:14 pm

XorA wrote:Found it, got confused with the SMD caps and fitted C97 with a 10uF, replaced with the 1nF and I have the magic K :-)
Well done Image Image Image

Another ZX80 lives :P

Mark

poglad
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Re: ZX80 Core - new ZX80 motherboards

Post by poglad » Sun May 18, 2014 8:46 pm

Oh - slowly understanding the schematic - it would be correct for those two to float without the ROM chip in. They would be either +5V or whatever the pin was connected to. 8-)

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1024MAK
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Re: ZX80 Core - new ZX80 motherboards

Post by 1024MAK » Sun May 18, 2014 8:56 pm

28C256 pin-out.png
28C256 pin-out.png (19.22 KiB) Viewed 1648 times
poglad wrote:I've done some more checking - with the ROM chip removed. In the various switch positions, with J100 connected in the right position, I've found that pin 27 /WE is high all the time, while pin 1 A14 toggles between low and floating in various positions. A13 and A12 toggle between low and high as I would expect.

If I connect J100 the other way (against the advice of the user guide), I can get /WE to go low sometimes. So I think those two pins are definitely the root of my troubles, and will look on the schematic and try to understand what's up.
Which version of the ZX80 Core is this? PokeMon's first version V1.0, or his V1.1?

As you refer to J100, I assume you have the V1.1 version.
For a 28C256, pin 27 (here /WE), the switch will / should override the /WR from the Z80 if the jumper J100 is correctly fitted to the right.

Your problem with pin 1 (here A14) does sound like the problem that presented itself on the earlier version of the board, but this is fixed with the V1.1 version.

So maybe the switch you have fitted (J5) is the wrong type? As it is this switch that drives/overrides the upper address pins A12, A13, A14 and /WR. In the correct position it supplies +5V to the correct address line and to the /WR line. Otherwise resistors drive the lines, ether to 0V directly, or to either logic high or logic low via the Z80 bus.

Mark

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1024MAK
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Re: ZX80 Core - new ZX80 motherboards

Post by 1024MAK » Sun May 18, 2014 9:01 pm

poglad wrote:Oh - slowly understanding the schematic - it would be correct for those two to float without the ROM chip in. They would be either +5V or whatever the pin was connected to. 8-)
The ROM (EEPROM) only drives the data bus (when selected by /OE and /CE both low). All other pins on an EEPROM are inputs only.

Mark

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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Sun May 18, 2014 11:43 pm

poglad wrote: If I connect J100 the other way (against the advice of the user guide), I can get /WE to go low sometimes. So I think those two pins are definitely the root of my troubles, and will look on the schematic and try to understand what's up.
I am pretty sure it is like described. Put J100 to the left position will give GND (over resistor) to /WR and this can be override through ROM switch in position 4-7 or C-F. On the right side you have the /WR signal at the EEPROM which should allow to write to the EEPROM (for programming) but this can be override with ROM switch too to write protect the EEPROM in position 4-7 and C-F. So this was meant to connect choose a ROM image when using the 32k or 64k EPROM (27C256 or 27C512).

Anyway it would be good to get into the schematics for better understanding.
A14 of EEPROM (pin 1) shouldn't float at all - there is a pulldown resistor which can be override via ROM switch to push it to High.
In the previous version V1.0a this has been left open accidently when not choosing position 8-F.

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Re: ZX80 Core - new ZX80 motherboards

Post by XorA » Tue May 20, 2014 2:57 pm

I am probably being really blind, but the Composite/Phono socket is missing from the parts list. I guess thats because it doesn't have a JXX number on the board.

G

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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Tue May 20, 2014 3:18 pm

Maybe a bit hidden after the modulator text (because it is an alternative): :mrgreen:
Miscellaneous:
1 x UM1233„IC99“ (ASTEC modulator UM1233, alternative chinch socket, print version)
1 x L9933uH/0,9R (only together with switched regulator IC97)1 x X1SFE 6.5 (ZF filter, 6.5 MHz, 3 pin, e.g. MURATA)
1 x ZX80Keyboard (ZX80 membrane keyboard, listed at sellmyretro.com)(alternative: 40 SMD tactile switches Alps SKQGAFE010, 0.98N, Farnell or RS-Comp.)
(alternative: 40 SMD tactile switches Schurter LSG 1301.xx)
1 x mini heatsink TO220 (for variant b with 74LS types and linear regulator)
1 x M3x8screw with nut (variant c)
1 x power supply 9V DC, 500mA, with audio jack 3.5mm
1 x cable 3-5 meter with 3.5mm audio jack, mono (!), alternative stereo with mono adapter on both ends

XorA
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Re: ZX80 Core - new ZX80 motherboards

Post by XorA » Tue May 20, 2014 3:30 pm

*goes off to find his white stick*

poglad
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Re: ZX80 Core - new ZX80 motherboards

Post by poglad » Wed May 21, 2014 2:22 pm

PokeMon wrote: Anyway it would be good to get into the schematics for better understanding.
I'm getting there. I'm holding off any further investigations until I've fitted the new filter/resonator and the ZX81 ROM. :)

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PokeMon
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Re: ZX80 Core - new ZX80 motherboards

Post by PokeMon » Wed May 21, 2014 2:40 pm

By the way - yesterday I "repaired" a newly buid up ZX80CORE here with no video signal.
I found, that the signal was created perfectly but forced down when setting the jumpers on J96.

He did set a jumper on J94 (for ZX8-CCB) which is not needed when SMD components are added on the backside and will connect the output of the two resistors R30/R32 directly to the video out / monitor which is supposed to be terminated with a 75R resistor. This advise appears in the V11 manual but is wrong. I have to correct it - so please check your board for this issue.

On the other hand his EEPROM lost data or data has been accidently overwritten while he placed one of the 74HCT157 in wrong way (turned 180°). So not sure but there is a 28C64 on the way to you freshly programmed for testing this.

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