Nominal video output signal on ULA pin 16 ZX81 rev 1?

[AMDeee]

Hi all, I have a ZX81 which displays only a white screen. I attempted a back porch mod and got no output, I'm using a scope to probe pin 16 of the ULA and I get a 15.6khz square wave formation with lows of 0v and highs of 4v. Is this expected from what the ULA should output? From what I've seen online the sync signal should not be 0v.

Thank you.

[msknight]

Was it working before the mod? (possibly a silly question if you had to do the mod to see an image in the first place)

My new and very limited experience with a white screen was that I had a short on the RAM lines.

I was given a lot of good advice on what to check in this thread...
https://www.sinclairzxworld.com/viewtop ... f=7&t=5300

[mrtinb]

From what I've seen online the sync signal should not be 0v.

It’s because the ZX81 does not follow the composite standard. It sends a signal where the difference from bottom of signal to top of signal is ok. Old TVs think this is ok. It does this to avoid making a more complex circuit that sends negative voltage. This is what I remember from the top of my head.

[AMDeee]

It was still a blank white screen before the mod, that's why I did it in the first place, just to check if the cursor was just washed out due to the missing back porch. From what I saw on the scope there doesn't appear to be a cursor being drawn else I think I should see a small dip in the waveform indicating that the black pixels of the cursor are being drawn? I'll give that link a look, thank you!

[AMDeee]

From what I've seen online the sync signal should not be 0v.

It’s because the ZX81 does not follow the composite standard. It sends a signal where the difference from bottom of signal to top of signal is ok. Old TVs think this is ok. It does this to avoid making a more complex circuit that sends negative voltage. This is what I remember from the top of my head.

Yes that appears to be the case, however from what I've seen online, the sync signal by the ZX81 is usually at around 1v for certain revisions and 230mv for others. Mine is strictly at 0v which I find strange

[1024MAK]

The ZX81 can't generate negative voltages, so the bottom of the sync pulse is indeed 0V or thereabouts. The black level is a higher voltage. Peek white is higher again.

The internal voltages (such as on ULA pin 16) are designed for the modulator and are not standard composite video signal levels.

For both the above reasons, a 100µF (although the actual value is not critical) capacitor is normally used to AC couple the signal, while blocking the DC levels.

Are these voltages with the input to the modulator still connected, or with it disconnected?

If you are only getting the sync pulses / a white screen, then at least some of the ULA is working. However, the Z80, the ROM or the RAM, or the communications between these has a problem.

The most common is a problem with the RAM, but its not always this.

Mark

[AMDeee]

I swapped the modulator with a 555 timer mod to generate the back porch, the readings I took where with the input to that circuit plugged in. I do suspect that something else is wrong on the board given that I see no attempt by the ULA to display black cursor text as you said. I guess next step is to see If there is any activity on RAM and if activity looks healthy. I just got a new scope so hopefully I'm able to fully troubleshoot it this time. If you have any advice on what I should check regarding RAM I'd be very grateful.

A good place to start is the link already provided to me I suppose, so I'll do that:

https://www.sinclairzxworld.com/viewtop ... f=7&t=5300

Thanks in advance!

[AMDeee]

Before I give the readings, I know for a fact that the z80 CPU is in working condition as I swapped it in another zx81 and it worked fine.

Here are some suspicious things I found:

18 /Halt ->

22 /WR ->

Address lines:

A0 to A7 look fine and active,

A8 to A15 Behave a little strange. All of them alternate from being LOW to having activity but as you go from A15 to A8 the frequency at which they alternate goes from around once per 2 seconds and increases. Not sure if this is normal.

Data lines:

All the Data pins look bad ->

Something along the lines of this or worse

_____________________________

/RAMCS on pin 18 and 20 of large RAM chip ->

/W on pin 21 of large RAM chip ->

______________________________

/ROMCS on pin 20 -> stuck HIGH, no pulsing.


/W on the RAM chip and /WR on the CPU seem to have the same exact waveform from what I can tell

[msknight]

If you don't have any RAM packs installed, I'm presuming there'd be nothing in the system to respond to A8 and higher? Just a thought ?

[1024MAK]

I'm busy at the moment, but...

As the display is created by a combination of software and hardware, if the /ROMCS is always high, then you won't get a display. It's the code in the ROM that produces the video image and the pixel data for the characters is also in the ROM.

The Z80 always produces a DRAM refresh address after each instruction cycle, this is the address on the address bus when /RFSH is low. A0 to A6 count up with each instruction cycle, A7 stays at whatever it was set to (normally logic 0) and A8 to A15 are the contents of another register, so should again stay the same most of the time. More detail: the refresh address is made up of the refresh (R) register and the interrupt (I) register. Refresh (R) register (bits 0 to 6) will continue to count from zero to 127, then back to zero again. Bit 7 will likely be at zero (it does not count, so stays at whatever value it was previously at). After each instruction fetch, the Z80 will put the refresh address on the address bus. A0 to A7 are from the R register. A8 to A15 are from the interrupt (I) register (because the Z80 always puts out a 16 bit address).

When no chips are actively driving the data-bus, each line will "float" between valid logic levels.

The Z80 will drive all address lines, regardless of what memory is installed. As the Z80 is likely to be executing garbage (because it's clearly not running the code from ROM), it's impossible to know what addresses it's putting on the address bus.

In a standard ZX81, the Z80 will see memory across it's entire address space. See here and here

Mark