ZXmore or the ZX80CORE follow-up

[Lurch666]

Had a thought and swapped the 74HCT245 around but that made no difference exept the black bar at the bottom has disappeared.swapped them back and the black bar is still missing.Don't know if this is significant or not.


Pulled the 74HCT245's and checked the ram and rom again but results were fine.The only possible problem is with the OE pins.I get the short and short high pulse but they are not stable on the scope-they flick forward and back but this could be because it's a 600ns repetition and I'm looking at them at 500ns.

Anyway thought I would post pictures of rom ram pins 22:(ram at the top)



and pins 24 (ram at the top again):



I can't seem to see what's wrong with the signals that's stopping me from getting a stable display.

[Lurch666]

Even though I'm not getting a proper picture I figured I might as well measure some of the outputs of the CPU to see what I'm actually getting:

Pin5/A15:

at 2ms I'm just getting a solid signal going from + to -



At 5us:




Pin17/NMI:

At 2ms I'm just getting a solid toggle:



and I had to take the frequency to 2us to get any sort of pattern:



Is it worth me checking checking pin 18 and 19?

[1024MAK]

Keep in mind that it is the machine code program (from the ROM) that actually generates the video picture.
Unlike computers which have a seperate video chip, if there is a fault with the ROM, RAM, CPU or nearly any of the support chips, ZX80/ZX81 based computers will not produce a stable display. Unlike computers which have video chips, where even with the CPU missing, often they will display some kind of static blank or garbage image.

Don't worry about the exact waveform on the address lines or data lines, as the signal depends on the current instruction that the CPU is reading or data that it is reading or writing. So unless it is running a very simple program, there may not be a clear pattern on most of these lines. The important things:- do the signals switch between 0V and +5V, and does the same signal line have the same signal at each chip it goes to.

Mark

[Lurch666]

Just checked A15 again and i was mistaken. I'm not getting any signal from it.its just background noise.if i try A14 im getting a clear pulse and A13 is giving a slower but still clear pluse.so for some reason I'm getting nothing on A15-it's stuck low.

[PokeMon]

Are you sure, this is at pin 17 (NMI) of CPU ?
Can you measure it with 50us ?



So the maximum frequency possible is the horizontal sync (64us / 15625 Hz) with 5us low and about 59us high.
Anyway it can not be faster. So this leads me to a possible short with some other signal.
This may be the reason - not sure as this may not been seen good in this resolution with 2ms.

Don't worry about the picture of the screen you posted - this doesn't look too bad. There are rolling pixel lines from top to bottom and the background has a faster pattern in my case. This is due to an open input of the pixel input.

Can you measure the horizontal frequency at pin 4 of IC6 and the vertical frequency at pin 10 of IC11 (which is possibly empty now in your setup) ?
Is the black bar in your screen photo constant (non-moving) ? Not horizontal and not vertical ?
Is the whole picture rolling or just some vertical pixel lines in the visible area ?

Mark is right, it is harder to measure in a live system as the signals are not that regularly like ROM and RAM access which derives depending what the computer is doing at the moment - and a moment is a few microseconds in this context.

[Lurch666]

Now I'm really puzzled. Rechecked pin 17 and now its just stuck high.maybe I has a short using the clip probe as i just held the probe by hand this time.

Pin 4 of ic6 is 50us frequency.
Pin 10 of ic11 is stuck high.
The black bar was stationery at the bottom of the screen but now its gone since i tried swapping the two chips.

[PokeMon]

Okay - I will prepare now a test rom.
You can use this for better investigation of single functions (like ROM read, RAM read, etc.) and will provide specific signals to measure as well which will make life hopefully easier.

[Lurch666]

I have an eprom burner and some eproms.I'm at work at the moment but will check what sort of eproms i have spare when I get home.

[PokeMon]

Can your software burn a SST39SF040 (flash rom) ?
This is the type installed in the ZXmore.
In fact it doesn't need a programming voltage and can be programmed and erase with just 5V power - so this is supported by many programmers. If this is supported I can email you the latest version or you can verify or reprogram the 512k ROM file (.bin) from the latest version provided here (1.8).

Important: This is pure binary and no intel hex format.

The first link contains the 512k rom file used:
http://sinclairzxworld.com/viewtopic.ph ... 832#p19174

Just to be sure the flash rom is programmed correctly.

[Lurch666]

Might have found the problem.tried to verify the rom and got errors so i wrote the 1.8 bin file to it and it verified fine.plugged it into the zxmore and replaced all the chips but got a static white screen.
So i removed the few chips so i could recheck with the scope but when i turned it on again i still had the white screen.no black rolling as i had before.
I decided to check the code in the rom again and it wouldn't verify again.
Hmmmm.
I wrote and verified the chip,shut down the computer,restarted again (without putting the rom onto the zxmore) and it failed to verify so I'm assuming the rom is faulty.will order a new one and see if it makes a difference.