New Gigatron in Russia

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macrisky
Posts: 5
Joined: 07 Sep 2023, 15:21

New Gigatron in Russia

Post by macrisky »

Hello All,

I have built my Gigatron from open-source Gerbers and schematics.
The first problem which I faced was fault in the gamepad subsystem. It was not the obvious problem,
but I have resolved it.
Symptoms: gamepad did not work, I could only see some random cursor movements right after starting the computer,
but it did not react to the buttons as expected.
Efforts: many hours spent with multimeter and multi-channel oscilloscope.
Resolution: the problem was with 74HC595 chip. These chips are manufactured by many different vendors.
The first vendor which I used was Siemens, the second was HGSEMI. They both did not work.
Finally, I took NES Semiconductor's 74HCT595 (Note: HCT, not HC) and now all works fine, including PS/2 adapter with
all keyboards which I have, even USB ones.
Explanation: Some 595 chips have 10 MHz resolution, some have 20 MHz, some have 25 MHz, it is noted in the corresponding datasheets.
For the Gigatron to work correctly with gamepad and keyboard, one should use at least 20 MHz chip.

Result: up and working Gigatron.

Last problem: Video image is shown with cut-off areas outside of VGA sync, like this:
img.jpg
img.jpg (172.14 KiB) Viewed 2655 times
Please help.
bmwtcu
Posts: 145
Joined: 01 Nov 2018, 12:02

Re: New Gigatron in Russia

Post by bmwtcu »

Is that a monitor or TV? If TV, does it have a overscan on/off or PC mode menu setting? Are you using 6.25MHz crystal or something different?
macrisky
Posts: 5
Joined: 07 Sep 2023, 15:21

Re: New Gigatron in Russia

Post by macrisky »

The monitor from the photo is the smart TV, but I tried simple VGA LED mini-monitor also, the result is the same. I thought about overscan, but did not find such option. I do not have tube monitor or TV.
The crystal is 6.25 MHz, but I cannot make sure that it's frequency is exact.
Hans61
Posts: 102
Joined: 29 Dec 2020, 16:15
Location: Saxonia
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Re: New Gigatron in Russia

Post by Hans61 »

I think it could just be the monitor/tv or quartz?
I have already tested Gigatrons on various monitors and TVs. If they synced badly, then the picture was shifted. I've never had a picture like this before, it's too big on all sides.

Why ROMv4? But it has nothing to do with the monitor.
bmwtcu
Posts: 145
Joined: 01 Nov 2018, 12:02

Re: New Gigatron in Russia

Post by bmwtcu »

I'm still leaning toward overscan. Most modern TVs I've used have a PC mode that removes all post-processing like scaling, etc. If you don't have that, I'd suggest trying a native 4:3 monitor (not TV) if possible. Something old enough where 640x480 was actually tested! Where I'm at, I've picked up many Dell 1907FP monitors for free locally since nobody wants them but me:P Is there a setting to force 16:9 aspect ratio on your TV? Is it just a stretched version of what you're showing or a fuller frame? Basically I'm curious if this is an artifact of converting to 4:3 on your particular TV.
macrisky
Posts: 5
Joined: 07 Sep 2023, 15:21

Re: New Gigatron in Russia

Post by macrisky »

bmwtcu wrote: 08 Sep 2023, 08:00 suggest trying a native 4:3 monitor (not TV) if possible. Something old enough where 640x480 was actually tested!
Switching between 4:3 and 16:9 aspect ratios does not help.
Will try to search for old monitor.
macrisky
Posts: 5
Joined: 07 Sep 2023, 15:21

Re: New Gigatron in Russia

Post by macrisky »

Hans61 wrote: 07 Sep 2023, 20:54 I think it could just be the monitor/tv or quartz?
I have already tested Gigatrons on various monitors and TVs. If they synced badly, then the picture was shifted. I've never had a picture like this before, it's too big on all sides.

Why ROMv4? But it has nothing to do with the monitor.
I figured out that VGA picture does not depend on crystal frequency. Now I soldered 7.352 MHz quartz. The computer itself works slightly faster, but video is still the same (has cut-offs).
Still thinking about searching for another old monitor.
walter
Site Admin
Posts: 161
Joined: 13 May 2018, 08:00

Re: New Gigatron in Russia

Post by walter »

The clock speed will not change the edge of the video, it will only make the Gigatron spit out the signal slower or faster. The more it deviates from 6.25MHz, the more trouble it will give the TV/monitor to sync to the image. But when sync fails, you will get nothing, not just part of the image. When the Gigatron is correctly working, all of the screen content is given out on VGA. It must therefore be in the TV/monitor.
macrisky
Posts: 5
Joined: 07 Sep 2023, 15:21

Re: New Gigatron in Russia

Post by macrisky »

walter wrote: 08 Sep 2023, 20:51 The clock speed will not change the edge of the video, it will only make the Gigatron spit out the signal slower or faster. The more it deviates from 6.25MHz, the more trouble it will give the TV/monitor to sync to the image. But when sync fails, you will get nothing, not just part of the image. When the Gigatron is correctly working, all of the screen content is given out on VGA. It must therefore be in the TV/monitor.
I tried to connect an old LCD monitor with VGA input. It has auto correction option (auto-sync) and the image is quite normal with it.
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