ZX80 Core - new ZX80 motherboards

[PokeMon]

Today I tested the NMI board (ZX80NMIX) and after a few modifications it's working quite good now.
I will try to reduce the number of components and not to make to complex.
I decided that the ZX8-CCB will be not part of the ZX80NMIX as people are anyway using the separate attached board ZX8-CCB on their ZX80CORE.

The board adds only the NMI pulses and will be switched off by default. This is important for use it with the original ZX80 ROM. It works only when enabled from the ZX81 ROM with switch NMI on command (activated with SLOW command or by default at startup of ZX81 ROM). This offers the opportunity to play nearly all available games on the ZX80CORE which are developed for the ZX81 in SLOW mode. Only a few games have their own display drivers and can be played using the FAST mode. Also the Forth ROM will run with NMI pulses only.

The additional board will be available in end of january.
Anybody interested may post here to see how many boards are needed.
I will offer more details after realization, I have to re-engineer the schematics and route the board.
After I can post an idea of price for this technical marvel.
The most important point for now is, that the NMI board works properly and will be available in the very near future.


By the way - the ZX80CORE first batch is run out now but I decided to do a second batch available approx. january 2014.
The ZX8-CCB will be integrated in this board on the solder side as smd components but harmless to solder with big items.
So I will use the SO package with 0.05 inch raster and resistors / capacitors in 1206 size.
I think even beginners could solder it - if not there is still the option to attach a ZX8-CCB board instead of self soldering.

[Mike3]

Great Count me in...

Mike

[fatbob_gb]

I am definately interested in one.

[balford]

Hey folks,

Having some problems building the ZX80Core and thought I might find some help here.

Upon power on, the /RESET line never goes about 1.4V - not enough to release the Z80 from reset. The VCC end of R21 measures 5V exactly, other end at 1.4V.
The system clock (6.5/3.25MHz) is running fine, not getting any sync signals or activity on A15 obviously.

I have a DMM and very basic USB DSO to help diagnose but not sure where to start.

The board had a very small short between GND and CLK on the edge connector when I got it, soon cleared though. The rest of the board has been gone over with a fine tooth comb checking for shorts.

Any ideas are very welcome!

B

[PokeMon]

The board had a very small short between GND and CLK on the edge connector when I got it, soon cleared though. The rest of the board has been gone over with a fine tooth comb checking for shorts.

Sorry for that. Maybe this happened during tinning. Was it directly at the edge connector ?

As the /RESET does not go up high, it is only a resistor and capacitor, typical R/C combination.

Maybe there is a short somewhere to some other signal which forces it to down. On the other hand, the resistor is 22k and this means, there is a small current flowing of about 0,15 mA. Every of the HCT types could sink easily quite more which would result in a lower voltage. Could be a short to a pulldown resistor with 10k as well, but there are only pull up resistors of 10k.

If your CPU is socketed, you should remove it and measure voltage again.
Could be maybe a wrong placed electrolytic capacitor C10 for reset circuit as well.
If CPU not socketed try just to remove the capacitor C10 and measure again.

[inaki]

Hi Pokemon.

Does the ZX80Core motherboard have the same physical layout, and mounting holes, as the original ZX80 ?
In other words, is it possible to place the ZX80 core inside an original ZX80 case ?

inaki.

[balford]

Thanks for the advice PokeMon!

Luckily everything on the board is socketed.

Removing the Z80 results in a nice reset signal with a rise time of about 0.5 secs.

Through a process of elimination I've worked out that removing IC15 allows the reset signal to be produced normally with the Z80 in place. I've swapped the chip with the other 74LS05 with the same results, so I think the IC's are fine. I've removed the IC15 socket to check for shorts, nothing obvious, and the CPU socket looks clear also.

Any other ideas? Grateful for your help by the way

Thanks!
B

[PokeMon]

Hmmm as there is a dependence of CPU and IC15 you could do following.
Pull out both ICs and measure the reset pin against all pins of cpu socket and against all pins of IC15.
So this are 39+16 pins to measure either a short or at least a resistor smaller 10k.
Could be a possibly short against CLK or /M1.
Both signals are affected by cpu and IC15.

[balford]

Thanks, made the measurements tonight and will try and grab some time with the schematic tomorrow or over the weekend to see what's out of place.

For reference, here's what I got:

pin 2 ic15
IC1 pin 6 1.46k
pin 10 24k
pin 11 1k
pin 17 11k
pin 22 4.27k
pin 24 11k
pin 25 11k
pin 26 22k
pin 29 3.94k

pin 3 ic15
IC1 pin 6 1.46k
pin 10 24k
pin 11 1k
pin 17 11k
pin 22 4.27k
pin 24 11k
pin 25 11k
pin 26 22k
pin 29 3.94k

pin 4 ic15
IC1 pin 13 0

pin 5 ic15
IC1 pin 6 1.46k
pin 10 24k
pin 11 1k
pin 17 11k
pin 22 4.27k
pin 24 11k
pin 25 11k
pin 26 22k
pin 29 3.94k

pin 6 ic15
IC1 pin 6 23.2k
pin 10 0
pin 17 32.8k
pin 22 26k
pin 24 32.8k
pin 25 32.8k
pin 26 44.5k
pin 29 25.8k

pin 7 ic15
IC1 pin 6 3.57k
pin 10 26k
pin 11 3.1k
pin 17 13k
pin 22 6.4k
pin 24 13k
pin 25 13k
pin 26 25k
pin 29 0

pin 8 ic15
IC1 pin 6 1.46k
pin 10 24k
pin 11 1k
pin 17 11k
pin 22 4.3k
pin 24 11k
pin 25 11k
pin 26 22k
pin 29 4.10k

pin 9 ic15
IC1 pin 27 0

pin 10 ic15
IC1 pin 6 0
pin 10 23.2k
pin 11 470
pin 17 10.4k
pin 22 3.7k
pin 24 10.4k
pin 25 10.4k
pin 26 22k
pin 29 3.53k

pin 12 ic15
IC1 pin 6 0
pin 10 23.2k
pin 11 470
pin 17 10.4k
pin 22 3.75k
pin 24 10.4k
pin 25 10.4k
pin 26 22k
pin 29 3.5k

pin 14 ic5
IC1 pin 6 470k
pin 10 22.8 k
pin 11 0
pin 17 10k
pin 22 3.2k
pin 24 10k
pin 25 10k
pin 26 22k
pin 29 3.5k

All others at least 0.5Mohm or greater.

B

[PokeMon]

Well this is very detailed.
I meant only IC1, pin 26 against the other 39 pins of IC1 and against the 14 pins of IC15.
Not every pin IC1 against every pin IC15.

Could be a short between two of the CPU pins as well because if you remove IC15 you have no clock at all. If you don't have a clock the most outputs of the CPU are floating / high impedance. On the other hand you could measure if you have 1.4V at any other pin on some IC when putting both ICs back in their socket.
Finally there could be a failure of IC1 (CPU). But just guessing for now.

You could try to either use another CPU and if you can not fix this issue on your board I can offer to find it for you when shipping it back to me, no problem.
It's not as easy on the distance as if I would have the board here, you know.

But first, good luck again !