ZX81 ULA

ZX80 / ZX81 hardware and software offered for sale or swapping
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Andy Rea
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Re: ZX81 ULA

Post by Andy Rea » Mon Jun 03, 2019 7:35 pm

well i have ordered more chips :D so thats one less task...

Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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mrtinb
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Re: ZX81 ULA

Post by mrtinb » Fri Aug 16, 2019 2:03 pm

Hi Andy :)

Just wondering. How does the 75 ohm pin affect the monitor? Will it destroy my monitor if this setting is wrong?
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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Andy Rea
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Location: notts UK

Re: ZX81 ULA

Post by Andy Rea » Fri Aug 16, 2019 3:38 pm

Hi.....

the link puts a 75 Ohm load resistor into the output path. it is meant for oddball monitors that have hi impedance inputs.

i highly doubt that any damage could occur if the link is set wrong. you most likely will get a wobbly image. as the monitor will not be able to "lock" onto the sync signals.

regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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1024MAK
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Re: ZX81 ULA

Post by 1024MAK » Fri Aug 16, 2019 4:03 pm

As long as you don’t connect a high voltage, it’s very unlikely that you will damage a monitor with a 50Hz or 60Hz vertical frequency and a 15625Hz horizontal frequency video signal or with a 5V logic signal from a logic chip.

All decent monitors/VDUs/televisions have some protection components fitted to the video input circuits that should prevent damage. It is less certain for very cheaply made consumer televisions.

Mark

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Re: ZX81 ULA

Post by Moggy » Fri Aug 16, 2019 4:22 pm

I have a small 5" portable B/W tv which unusually has a composite input but for some reason the 75ohm thing seems absent. If I use say the new ULA or pokemons video board the picture is too bright to watch but with the ULA jumper set to 75ohm the picture is perfect so in my case this is a very useful feature.
My point being my unprotected TV hasn't suffered any damaged when initially run without the jumper setting so as Mark says the kind of signal from the ULA shouldn't damage anything.

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1024MAK
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Re: ZX81 ULA

Post by 1024MAK » Fri Aug 16, 2019 5:55 pm

The “standard” that has developed is for composite video (also known as baseband video) to be fed via co-ax cables that have a characteristic impedance of about 75Ω, hence most have an input resistance of 75Ω (as when dealing with video frequencies, every part of a circuit should have the same impedance to prevent ghosting/reflections).

However, some equipment (broadcast quality monitors, industrial monitors and other professional equipment) may have pass through connectors and a termination switch. When the termination switch is on, the input is matched to 75Ω.

But some equipment does not have any termination resistance internally. Hence, if no separate terminator is used, the signal voltages will be different.

There are other problems, but that’s another subject for another day...

Mark

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