ZX81+38 Build Help

[DeadTed]

Hi All, I compiled the below attached using my oscilloscope. I compared all the Z80 pins between an original zx81 and the zx81+38.

[DeadTed]

@Fdiskitup - Thank you for the suggestions and the rom images. I removed the rom but still blank screen. Your rom image is the same as mine but I went ahead and burnt a copy of yours anyway and still nothing. the 60mhz is set for PAL as I am in Australia

[DeadTed]

@Fdiskitup, are you able to post a picture of the Botton of your board, I would like. to see how your jumpers are configured.

[Fdiskitup]

So in short I started with a vintage 24 pin 8K PROM and changed to a "modern" 28 Pin 32k EEPROM.

My TS1000 ROM, i.e. the Vintage 24 pin Rom says "Sinclair Research on the top (8223PD, D2364 649 (c) 1981). It was a known good chip in the original TS1000 board, when tested with a good ULA (Ferranti ULA2C210E, 8310), However, the original ULA was Ferranti ULA 2C184E 8223 which appears to have failed (black screen even after the simple transistor composite mod). Anyway not wanting to Hijack the thread back to ZX81+38 build..

With the TS1000 ROM I set the jumpers JP8,9,10,11 and cut the traces under the board. But no joy, I got a black screen on an LCD panel tv (dual PAL / NTSC). Did I miss something ? was I supposed to set JP1 to 8K ?

I used BIN Wizard (a software provided by Damian Woroch) to stitch 4 versions of the 8k rom (version 2) into one 32k bin file.
Changing the ROM Chip to an EEPROM SST27256 with 4 copies of the version 2 ROM, then changing all the jumpers JP8,9,10,11 back to the original defaults did give me a bootable machine.

[Fdiskitup]

the 60mhz is set for PAL as I am in Australia

is it not 50Hz for PAL and 60Hz for NTSC ?

try lifting one side of that jumper cable out of the board ?

I can send pics of the jumpers when I get home - so tomorrow for you.

[1024MAK]

To be pedantic, it’s not PAL, as PAL is a colour encoding system. PAL in the U.K. uses CCIR system I, but PAL can be used with other TV signal standards. similarly, it’s possible to use the NTSC colour encoding system on 625 line 50Hz systems.

The U.K. TV system is 625 line, 15625Hz horizontal (line) frequency, 50Hz vertical (field), also known as ITU CCIR system I (I for India).

The U.S.A. TV system is NTSC 525 line, 15750Hz (monochrome standard or 15734Hz for colour) horizontal (line) frequency, 60Hz vertical (field), also known as ITU CCIR system M.

A ZX81/TS1000 (or clone) can (normally) be configured to run either 625 line, 50Hz OR 525 line, 60Hz. However, not all machine code software takes notice of the hardware setting, as the display is actually software generated.

Mark

[DeadTed]

To be honest, my TV supports both 50 and 60hz automatically, so I am sure that is not the problem, especially given that if you look at the comparison table I posted, clearly there are a few signals to the z80 missing. only thing I can think of is one of the logic chips is no good as I tested the z80 in my original zx81 and its fine or I have not set the jumpers correctly.

[Fdiskitup]

jumpers front 2.jpg

jumpers rear 3.jpg

Here are the jumper pics, but unless you cut some traces you should be good just leaving the defaults. For JP8, 9, 10, 11 the board is already set for a 27C256

with my scope on the CPU I see pin 1 to 10 - waveforms (importantly pin 6 is 3.23 MHz "the clock") - pin 2 of U21(HC04) should have 13.00 MHz, pin 4 of U7 (393) should be the clock.
you expect pin 11 to be high (5V)
pins 12 to 20 - activity (17 slow, 20 slow)
pins 23, 25, 26 have nothing
pin 29 is low (its the GND)
everything else shows activity.

[Fdiskitup]

Hold on, do you have a voltage on pin 11 of the CPU ? in your table it says Nothing ! there must be something on it for you to get all the waveform activity....
Anyway I'm concerned you've got nothing on address line pin 37, A7.. might be a symptom rather than a cause.

are you getting 5V power from a USB charger or a PC USB port ? (I failed to power this properly with a USB Li-ion backup battery charger).

[1024MAK]

Use the attachments tab under the edit box to add photos.

Note that the forum does have file size limits. So if needed, reduce the file size using a image editor.

Mark