ZX81CCP video output for ZX81

[1024MAK]

I don't think the ZX81 generates the full TV sync.

[PokeMon]

One question - you may have answered this already - what happens during SAVE/LOAD from tape??

C

Well better measure. So it is really generating sync pulses (pull output down to sync level). So what to do, what you think ?

I found short following sync pulses during LOAD with approximately 20us cycle length - so about 50 kHz. When data is incoming it is maybe not fixed to that cycle length. But can not find out better with a classic scope. Don't have a scope with memory.

During SAVE output is hold at sync level for about 5 seconds, after pending between min (sync level) and max output. Not sure what frequencies are generated here, difficult to measure again. Maybe could calculate if from the ROM routines.

So does maybe anybody here has a digital scope with memory and could measure out the min/max cycle length (high period) between sync pulses generated by SAVE or LOAD ? I was a bit confused, that sync low (pulse) is exactly 5us like from the sync generator. Maybe the sync generator is just switched on to give something like a picture and mixed with the incoming data. Not sure.

I think it could be fixed in that way, that too short periods between sync pulses could be suppressed, so that this would not be a valid sync pulse. So you would maybe have a stable picture during LOAD. I am not sure if that could be done during SAVE in same way. Maybe it is more common to load programs to ZX81 than to save from ZX81 to wherever. I don't think that people develop or work on real Zeddy, mainly load programs from PC to Zeddy.

So what you think about ?

[sirmorris]

The sync signal is also the tape out signal.

http://www.wearmouth.demon.co.uk/zx81.htm#L031E

IIRC the horizontal sync signal is also present in the SAVEd file...

During LOAD the vsync is manipulated manually to show the loading bars.

Sorry if my simplistic explanation isn't what you were asking; it's early and my brain is slow

[PokeMon]

Well the question was, what you would expect how ZX81SCP should handle video out during SAVE and LOAD.
I think that was your question about it a few days ago.

[sirmorris]

I was just thinking out loud. As SAVE uses the vsync signal I wondered if the additional sync signals would superimpose and spoil the SAVE. It's not a problem for me though

[PokeMon]

Well - I think it would be nice to have a synced picture during load as this could exceed longer times and maybe often used if you are not owner of some additional hardware like ZXpand.

So made a small extension which should help to trigger only sync pulses with a distance of approx. 44 us.
Need one buffer, one resistor and one capacitor additionally.

Zwischenablage44.gif (21.06 KiB) Viewed 6826 times

So I will try this out. If it's good I would implement it.

Need my dev board back, Paul.

[gozzo]

Could this 'standby/FAST sync generator' function be programmed into the ZX ULA replacement modules? If so -

[Paul]

Need my dev board back, Paul.

I'm going to send it back tomorrow as today I will be working too long

[PokeMon]

Need my dev board back, Paul.

I'm going to send it back tomorrow as today I will be working too long

At this high temperature today ?

[PokeMon]

Could this 'standby/FAST sync generator' function be programmed into the ZX ULA replacement modules? If so -

Yes of course it could be done.
I think Andy could do this easily as the ULA is creating the sync signal.