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 »

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

Andy
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mrtinb
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Re: ZX81 ULA

Post by mrtinb »

Hi Andy :)

Just wondering. How does the 75 ohm pin affect the monitor? Will it destroy my monitor if this setting is wrong?
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Andy Rea
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Re: ZX81 ULA

Post by Andy Rea »

Hi.....

the link puts a 75 Ohm load resistor into across 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
Last edited by Andy Rea on Mon Aug 19, 2019 12:50 pm, edited 1 time in total.
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1024MAK
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Re: ZX81 ULA

Post by 1024MAK »

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 »

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 »

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...

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Re: ZX81 ULA

Post by Bloodnok »

1024MAK wrote: 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
Spot on Mark. In a past life I worked as an engineer in the TV industry and 75Ω terminations were the standard for broadcast equipment. This allowed the same video signal to be looped through through several devices before being finally terminated at the last one saving cabling and the cost of individual video amplifiers.

The vLA81 has a low impedance video output which will provide a nominal 1Vp-p into both unterminated (Hi Z) or 75Ω inputs. There is no need to add a series 75Ω resistor to lower the video amplitude for unterminated inputs. In 75Ω mode (SW1 in the off position) the output is also clamped to 0V during sync so that if capacitive coupling is used the syncs don't get clobbered by the capacitor charging up.

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Re: ZX81 ULA

Post by dognaldo »

Andy, sorry to dig this topic, but I need to buy an ALU for my ZX81 clone. Do you have one to sell?
Thank you.
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