Voltage output range for TZXDuino? aka trying to get it wokring

Discussions about Sinclair ZX80 and ZX81 Hardware
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gwpt
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Joined: Mon Feb 01, 2021 7:01 am

Voltage output range for TZXDuino? aka trying to get it wokring

Post by gwpt »

Hi,
I have built a TZXDuino wanted to verify what the output range should be.
Mine seems to swing between around 2.5V and 4.1V which seems a bit odd.
before I go opening my zx81 (I've never used it with any kind of storage before, so not sure if it works) I wanted to make sure the TZXDuino was good first.

BTW The symptom i get when loading is (I have put a composite out mod in my zx81 and am running it into an LCD TV), I get the black and white verticals for a few seconds, then the screen goes black. Then I press play on the TZXDuino, I get the bars for a bit (ranging from a split second to a several depending on the .P file) and then I lose all video signal. I have to reboot to get the video back.

Thanks
Trevor_B
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Re: Voltage output range for TZXDuino? aka trying to get it wokring

Post by Trevor_B »

I was having similar issues loading from a TZXDuino on my Minstrel 3, and it might actually be working but your screen can't cope.

My Minstrel runs via composite video - and one of the guys on here (Mark or Moggy) informed me that the loading screen tends to cause issues with modern screens - causing them to lock (sorry guys - I know you can explain this better..!). I'd put an amplifier in line with the txduino output to boost the audio signal too - still had the same thing.

I ended up buying a cheap B&W telly with composite input from Ebay (cost me £10 inc delivery!) - this proved that it was actually loading.


Trev
gwpt
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Re: Voltage output range for TZXDuino? aka trying to get it wokring

Post by gwpt »

Thanks, Trev. the old monitor is a good idea, sadly I've got rid of my old CRTs! I'll have a hunt.
(As I DIYed the tzxduino I am still curious what voltage swing I should be getting. Just want to make sure it's not my build)
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1024MAK
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Re: Voltage output range for TZXDuino? aka trying to get it wokring

Post by 1024MAK »

Modern LCD (and the later CRT) TVs get all upset if they don’t receive the correct synchronisation video signals.
When a ZX80/ZX81/TS1000/TS1500 or clone is loading or saving, it does not produce any valid video synchronisation signals.

Rather than “rebooting” try turning the TV off after you think loading has stopped, wait ten seconds, then turn it back on.

If the computer has crashed because the loading signal was too low (weak) or too high, often the computer will crash. Then you will need to restart it...

An older CRT TV just does the best it can (it’s really dumb), so you get the weird stripes.

ZX80/ZX81/TS1000/TS1500 and clone computers are effectively rather deaf. Remember they were designed to work with the old shoebox cassette tape decks, that just rerouted the internal loud speaker output to the earphone jack. Hence anything that tries to feed a Ear socket on a ZX80/ZX81/TS1000/TS1500, ZX Spectrum or a clone needs to have signal that has a voltage level that is high enough. A voltage of up to 5V is fine.

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AsaGong
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Re: Voltage output range for TZXDuino? aka trying to get it wokring

Post by AsaGong »

Hello....I don't think you are detecting both zero crossing points in the full cycle. To detect both you will have to feed the input of the H11A1 AC via a full wave rectifier, so both positive and negative cycles conduct through it.
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