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Theoretically, if I would wire my CGA card output to a proper display using R,G,B,Hsync,Vsync,Ground lines only, I should see something sensible on the screen. I would lose Intesity information, all 'bright' colors would look the same as 'dark' colors.

The requirement is of course that the display is able to support H and V frequencies. It's a flat-screen TV in my case with VGA input and is able to handle 15kHz.

The signal from CGA is 'TTL-level', about 5V. VGA signal is about 1V.

Can this overvoltage cause damage to the TV-set?

Should I use resistors on the lines and what value?

UPDATE: this would work all the same for EGA, since it's conceptually the same just with dedicated Intensity for each color. In this case the color degradation would be more noticable. Is this correct?

UPDATE: There are multiple reasons for this not to work. One is low amperage. One more is that EGA mode uses reverse on VSync line. Now I can see why so many people are selling complex converter circuitry. I will need to buy one.

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  • On old Samsung CRT monitors there are zeners on rgb inputs, but this is definitely not an answer if other displays will survive 5v.
    – Vlad
    Oct 19, 2023 at 9:16
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    You may want to have a look at this answer.
    – Raffzahn
    Oct 19, 2023 at 10:28
  • Found this schematics in the linked answer: electroschematics.com/cga-scart-adapter - it does handle Intensity which is bonus and reminded me to connect Ground pins otherwise I will see nothing. But I am confused about the BS170 MOSFET and the wizardry around it. Why not just connect HSync to HSync?
    – Dercsár
    Oct 19, 2023 at 14:13
  • Ok, that transistor is there to mix H and V for SCART, I don't need that part
    – Dercsár
    Oct 19, 2023 at 14:28
  • 3
    Does this answer your question? How can I adapt a digital CGA/C128 signal (RGBI DE-9) to a modern monitor?
    – Raffzahn
    Oct 19, 2023 at 15:46

2 Answers 2

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For the actual questions, yes the overvoltage could cause damage, and yes you should use resistors. The problem is, TTL outputs are not meant to be connected to analog VGA inputs, not even with resistors.

CGA uses TTL outputs for RGBI and HV signals. The signals are buffered by a 74LS244 line driver.

VGA input uses 75 ohm termination for the analog RGB lines, and HV sync inputs are TTL compatible digital inputs. The RGB voltage 100% level is 0.7V into the 75 ohms, or 9.3mA.

So the sync signals are electrically compatible, no reason to change that. They can be directly connected.

The problem is that a TTL output cannot be expected to drive out 9.3mA into any load. A modern LS244 buffer, which by the way has much stronger output drive ability than standard LS TTL chips that are not buffers, is specified to drive out only 3mA, and the only guarantee you get is that the voltage is at least 2.4V, or 3.4V typically.

So while the theory is that CGA outputs 5V logic signals and can be divided down to 0.7V through a 461 ohm resistor sounds good, there is no reason to assume it will work or even survive without damage.

The CGA TTL output is not intended to drive 536 ohm load. It can't drive the required 9.3mA to VGA analog input, at least not without serious overload and unpredictable output voltage, so at least the series resistor should be much lower than 461 ohms in practice.

Another problem is that a VGA monitor will likely expect VGA resolutions only and may not lock on to any signal it does not recognize from the list of supported formats. However if this is a TV with a SCART input, it might share the analog input so it could understand TV resolutions from VGA input as well, but do not expect it to work.

VGA is a video signal with about 31 kHz horizontal rate, 70 or 60 Hz refresh rate depending on resolution. CGA is a 15 kHz video signal more closely related to 60 Hz 525-line interlaced TV scanning system, but it is not interlaced but progressive scan.

Many digital TVs don't actually expect such a low format and even if it works with standard TV formats it may not handle the 200-line progessive 60 Hz. Since the TV has a SCART input, it indicates support for 50 Hz TV formats so it may have poor or no support for 60 Hz TV formats.

Edit:

Same for EGA. Uses same LS244 logic buffer. EGA video has 22 kHz horizontal frequency. Unlikely that TVs or monitors would lock on to the signal as it's practically never used on VGA connector.

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  • There are VGA monitors that will happily synchronize with a 15kHz horizontal rate, a list is here: 15khz.wikidot.com
    – tofro
    Oct 19, 2023 at 17:31
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    @tofro I am not denying existence of such monitors. Just saying that it is more likely that the picture quality is not very good, if the monitor even bothers to show any 15 kHz resolutions at all.
    – Justme
    Oct 19, 2023 at 17:59
  • I have never thought about not having enough amperage. Thanks for the detailed explanation.
    – Dercsár
    Oct 20, 2023 at 10:30
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The TV set probably has some protection against overvoltage, but you shouldn't rely on it. Protection can suffer stress and eventually fail if you continuously run stuff out-of-spec like that.

SCART has a nominal impedance of 75 ohms and the RGB signal levels are 0.7V peak and composite sync is 1V peak. (See e.g. http://martin.hinner.info/vga/scart.html.) So a wheeze back in the day to convert RGB video from a Beeb (which also had TTL video) was to insert resistors in the plug to form a potential divider using the 75 ohm resistor in the TV. A quick bit of maths suggests a 470 ohm resistor would be suitable for the RGB lines and 330 ohm for the sync.

Actual electrical engineers will be appalled at this because the output impedance is wrong, but it should be fine for the three feet running from the computer to the telly.

Unfortunately for you, CGA appears to have separate V- and H-sync and you can't just turn those into composite sync with passive components. However if this is a genuine CGA card with composite video output there is one final wheeze. SCART RGB takes its sync from the composite input, so if you have a genuine CGA card with composite output there's a fairly good chance that you can feed that to the SCART composite input and it'll work. You will not need a resistor here as the signal is already at the right level.

You may also need a pull-up on SCART pin 15 to tell it to take colour information from the RGB lines rather than the composite signal, although some tellies can be explicitly told via their menus that it should prefer RGB over composite. CGA doesn't provide power on its DE-9 connector, but you can steal it from the lightpen or modulator connector (the latter is tempting because it also has composite video on it so that's one less place to attach this FrankenCable) but you'll once again need a potential divider to get it to the 1–3V required. 270 ohm feels about right.

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  • Thanks for your explanation. I wish to use VGA, not SCART. (OP updated) In VGA, there is separate V and H signal, so I should be fine, using 330 ohm resistors on those lines.
    – Dercsár
    Oct 19, 2023 at 13:49
  • @Dercsár I don't see why you would need 330 ohms on sync lines, so that is likely an incorrect idea.
    – Justme
    Oct 19, 2023 at 13:57
  • According to this schematics: electroschematics.com/cga-scart-adapter I need 470 ohm resistors on R,G,B and H,V lines and should be fine.
    – Dercsár
    Oct 19, 2023 at 14:31
  • @Dercsár It's just a random circuit on the Internet without any credibility. It won't work a bit, mostly because on a CGA card the pin 7 does not output a supply voltage for the circuit, it is not connected and some diagrams show pin 7 is a signal from the monitor but it is not connected in monitor schematics either. And you also need a MOSFET so simply resistors won't do. The sync voltage would be too high, it must be about 0.3V, not 1V. In short, that won't work, and even if you apply external 5V supply, it is still a poor circuit.
    – Justme
    Oct 19, 2023 at 17:52
  • I suppose hacks used to be called wheezes?
    – user253751
    Oct 20, 2023 at 14:28

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