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Thread: Looking for insight into video design

  1. #1
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    Default Looking for insight into video design

    So I'm pretty close, I think, to solving my 'no text's situation with this Hyperion. I'm just trying to understand the circuit a little better. The datasheet for the SMC 8021 says you can connect video output right to the VID pin, and I have verified that you can and that it produces text there.

    So I'm wondering why you need to have a couple of 7474s, a 7408 and two 74157s downstream from the 8021's VID pin before it gets to video out. VID goes to the D of a 7474, the Q of that 7474 goes is ANDed with something in a 7408, and then out to a 74157, and then to a 74244, one half of which goes to composite video out, the other to 'video out'

    Is that something to do with switching between text and video modes? Ie so that text is suppressed when graphics mode is active?

  2. #2

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    The spec sheet I saw says you can connect it to a video input. This is not a composite video though, if that is what you want.
    Dwight

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    No - but I did connect composite to that pin anyway, and could clearly see text being output. So I'm reasonably confident that is working.

    I have traced the circuit further down though, and have found a potential problem with the 74LS08 at U52. You can see in the pic here that the VID signal from the SMC passed through the 7474 and then is ANDed using two gates on that chip. The second AND gate seems to be working fine, but there is zero output on pin 11, even though there is signal on pins 13 and 12.

    hyperionvidprob.jpg

    Here's a larger version - the forum shrinks pictures too much which is why i avoid using it. https://drive.google.com/file/d/1FqH...ew?usp=sharing

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    So I removed and installed another 7408. Now I am getting no text at all and a cursor that immediately is placed mid screen. The machine will not try to boot.

    I checked the EPROM... there is activity going in but nothing on the outputs.

    I'm concerned this board may be more than 2 layer. When I was removing the 7408 one of the pads broke off. The pad on that side wasn't connected to anything, and the pad on the other side appeared to be ok.

    I'm looking at the datasheet for the 2732 and I don't see anything that could be acting on it to stop it from outputting data. Not sure what the deal is here.

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    Is there anything that activates or shuts off a 2732 internally? Or if you see data going in, does that mean there should always be data coming out? I'm seeing data going in no problem on all the address lines, but zero coming out. 2732 tests okay in my reader.

  6. #6

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    There is a chip enable and and output enable. Both need to be zero at the same time.
    Dwight

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    I am seeing vias that go from one side to the other but appear to go nowhere from there.. is that possibly a sign that this board has more than 2 layers?

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    Yeah, darn it.. I think this board has more than 2 layers. I think when one of the vias in the 7408 socket broke, it broke a connection between that pin and wherever it goes. At this point i'm thinking it's a pulldown on one of the unused AND gates.

  9. #9

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    You have a schematic, ohm things out. A broken trace can be replaced with a wire.
    Dwight

  10. #10
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    For sure. One thing I'm not clear on.. when you see on a schematic multiple connections coming together as one and then going out from there collectively to another page.. does that mean they are electrically connected, or is that just a convenient way of getting them all to the next drawing?

    Like say I have a connection from Pin 1 of one IC and then another coming from pin 2 of another IC.. the schematic will have to two lines come together and then go off as one line to another page.

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