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Paranoia: any chance P2 3.3V GPIO will fry VGA monitors by mistake? — Parallax Forums

Paranoia: any chance P2 3.3V GPIO will fry VGA monitors by mistake?

So unlike the typical resistive video DACs people use for generating analog RGB VGA in the past on P1's, it appears that the P2 is designed to natively support driving VGA RGB directly from its IO pins which can be configured to output up to 2V analog voltage levels via a 75 ohm source resistance, which becomes 1V into the 75 ohm terminated load in the VGA monitor itself.

One thing I was wondering here is whether in general VGA is designed to “safely” accept up to 3.3V on its RGB inputs in case of any errant condition where 3.3V is directly being output to any P2 GPIO pins attached to VGA devices, instead of having the RGB pin’s modes safely setup as 2V DACs with 75 ohm impedance at all times. I think this type of pin setup problem could quite easily happen during application development if you happen to select the wrong P2 pin or pin mode settings by mistake in your code or if you happen to try out some other P2 code from elsewhere that just drives out 3.3V on the IO pins assigned to VGA on your board, before you realise it. This was never an issue for the P1 VGA with its resistive DAC, but perhaps it now might become one for the P2?

I expect the outcome of such a condition will totally depend on the VGA monitor you have and whether its has sufficient input protection so any over-voltages outside the normal 0-1V range expected (or 0 - 0.7V perhaps?) would be clipped (or otherwise safely accepted without damage). Because VGA came about when 5V was king and there are other 5V/TTL level capable signals used for sync and I2C/DDC EPROM reading functions also present in the VGA cable I’d sort of hope VGA was originally designed to cope with possible shorts that get up to 5V onto their RGB signals and protect against it, but of course I don’t really know this for sure. Maybe some cheaper or modern day VGA monitors might skimp on this input protection and could be fried if they ever see raw 3.3V right out of the P2 at their inputs? It’s only going to be able to get 44mA running through 75ohms if driven at 3.3V (we are not talking hundreds of mA) but I wonder if that higher voltage might mess up the monitors ADCs if it’s sustained.

So if you are paranoid and wanted to restrict this problem in the first place, would there be a good design practice to somehow add a little extra output circuitry to any P2 VGA PCB designs to limit this possibility, and if so what could it look like? I was thinking perhaps some type of Zener diode connected to ground in parallel to the VGA load and clamping at something a little above a 1V breakdown voltage for each RGB signal line might help, but whether or not that would stress the P2 GPIO pins without additional current limiting resistors when they inadvertently try to drive 3.3V into these Zeners is not known to me…. and adding extra current limiting resistors to deal with that could go mess up the nice 75 ohm impedance. Would the internal P-channel FET resistance of 19 ohms or thereabouts be enough to limit the P2 pin from burning out if its 3.3V level IO pins get shorted to 1V with such Zener protection or is something else required?

Any thoughts? Is this even worth worrying about?

Comments

  • Don't worry about it, I just don't think its an issue.

    Keep in mind P8x32A's have been driving 3v3 into VSYNC and HSYNC monitor inputs for many years...

    If you do want to clamp, leds can have much sharper 'knees' than zeners.
  • roglohrogloh Posts: 5,758
    edited 2018-10-16 03:26
    Tubular wrote: »
    Don't worry about it, I just don't think its an issue.

    Keep in mind P8x32A's have been driving 3v3 into VSYNC and HSYNC monitor inputs for many years...

    If you do want to clamp, leds can have much sharper 'knees' than zeners.

    Yeah I guess you mean IR LEDs as I think the IR ones start a little above a volt which would be good. The visible ones would start to clamp at higher voltages like 1.8V etc for red which might be already getting high.

    Hopefully this is not an issue, was just thinking about it when laying out a board and running the raw RGB signal lines directly from the P2 output pins to the VGA connector and had a small dose of paranoia wondering if this might be an issue. I do know the sync lines do safely accept the full TTL range, it's the RGB pins that desire 0-1V or so.

    If I had a sacrificial monitor lying about it would be good to feed it 3.3V to see what "whiter than white" actually looks like. :smile:
  • jmgjmg Posts: 15,171
    rogloh wrote: »
    Is this even worth worrying about?

    I doubt it's a real issue, ( 5V might be more of a concern...) plus it's unlikely to be a long duration oops.

    rogloh wrote: »
    .. I was thinking perhaps some type of Zener diode connected to ground in parallel to the VGA load and clamping at something a little above a 1V breakdown voltage for each RGB signal line might help...

    Zeners have quite high C, and are soft at low volatges, and seem to cease at ~1.8V (1v/0.75V 'zeners' are diodes)
    A simple diode string could be sufficient, if you really wanted to do something...

  • cgraceycgracey Posts: 14,134
    I think the 75 ohms is only to get the signal down the cable. After that, I would imagine there is some current limiting resistor before going into active circuitry.
  • roglohrogloh Posts: 5,758
    edited 2018-10-16 04:16
    Can't recall too much of my transmission line theory these days but to stop reflections I thought the VGA cable ideally needs to be terminated with 75 ohms as well inside the VGA monitor. If so wouldn't that 75 ohms basically be the only current limiting resistor, at least for pure DC voltages being input? The developed voltage over the 75 ohms then gets amplified or sampled to then drive the CRT/LCD electronics. But if the input voltage is too high above 1V it might cause issues for the monitor, depending on the amount of clamping or other protection present. That was my main concern.

    Yes maybe there could/should be additional current limiting resistance after the voltage across the 75ohm resistor is sensed and tapped into the rest of the amplification circuitry in case the voltage is too high.
  • cgraceycgracey Posts: 14,134
    The monitor has 75 ohms resistance to ground on the color signals. I would think that cable/resistor junction would then go through a series resistor on the way to some amplifier or buffer. Highly unlikely even 5 volts would hurt anything.
  • Yeah, I'd hope so too. It's basically just some design paranoia, hence the thread title.
  • jmgjmg Posts: 15,171
    rogloh wrote: »
    Yeah, I'd hope so too. It's basically just some design paranoia, hence the thread title.

    I think Chip did mention the DAC's worked ok down to 2.1V, so that's another possible means to do protection.
    It may be a simple Series R (or even PTC ) in that blocks Vio can give a primitive short/ miss config protection.
  • kwinnkwinn Posts: 8,697
    On a few occasions I have had customers connect their VGA monitors to the instrument interface connector (5V TTL) without doing any damage to either one.
  • jmg wrote: »
    I think Chip did mention the DAC's worked ok down to 2.1V, so that's another possible means to do protection.
    It may be a simple Series R (or even PTC ) in that blocks Vio can give a primitive short/ miss config protection.

    Yeah I'd wondered about that possibility too jmg. The good thing is that each Vio drives 4 pins which works out nice for doing RGBS or just RGB (SyncOnGreen). Would at least go down from 3.3V to 2.1V, but I think I'm probably going to ignore this issue, for now anyway.

    By the way does RGB with a sync on green output still work okay with the known bug in the colorspace converter? Has anyone tried it yet on a real P2? I do see the sync component requires an addition in the formula for FI, FY and FQ and wasn't that the part that was broken with the $signed thing? Presumably the separate H sync (or more practical a combined H+V sync) method would also work in that case?

  • cgraceycgracey Posts: 14,134
    rogloh wrote: »
    jmg wrote: »
    I think Chip did mention the DAC's worked ok down to 2.1V, so that's another possible means to do protection.
    It may be a simple Series R (or even PTC ) in that blocks Vio can give a primitive short/ miss config protection.

    Yeah I'd wondered about that possibility too jmg. The good thing is that each Vio drives 4 pins which works out nice for doing RGBS or just RGB (SyncOnGreen). Would at least go down from 3.3V to 2.1V, but I think I'm probably going to ignore this issue, for now anyway.

    By the way does RGB with a sync on green output still work okay with the known bug in the colorspace converter? Has anyone tried it yet on a real P2? I do see the sync component requires an addition in the formula for FI, FY and FQ and wasn't that the part that was broken with the $signed thing? Presumably the separate H sync (or more practical a combined H+V sync) method would also work in that case?

    Colorspace conversion works fine for things like Y-Pb-Pr. It's just modulation, like NTSC uses for color that doesn't work on the current silicon.
  • rogloh wrote: »
    Any thoughts? Is this even worth worrying about?

    This is a schematic for an IBM 6543 SVGA monitor.
    https://elektrotanya.com/ibm_6543svga,g50_sch.pdf/download.html


    As you can see, the RGB lines have a capacitor rated at 50V and a 390ohm resistor in series with them.
  • Good find. That cap will block any sustained 3.3V nicely. Seems IBM protected their monitors, hopefully the copiers copied it.
  • MIchael_MichalskiMIchael_Michalski Posts: 138
    edited 2018-10-23 18:13
    Heres another from an amstrad SVGA monitor. (This kinda takes me back to the good old days. I used to pick up extra cash by going around to all the local computer stores and fixing CRT monitors. I remember thermost common problems. Often the solder joints on the socket on the back of the CRT would crack from thermal stresses, or the solder joints on the flyback transformer. Other common ones were that the color drivert (usually located on the board on the back of the CRT) would burn, or the horizontal output transistor or driver IC would burn. It would always be so impressive to the shop owners when I said I could fix bad monitors and they would give me six dead monitors to see if I was legit, and Id have three running in 10 minutes because I'd checked those four things. Of course those other three would be the ones that could take hours. If I was lucky, I could pull a color driver or output transistor from the others and fix the easy ones. Back then we didn't have as many parts suppliers like we do on the internet these days, so parts were hard to find and crazy expensive (like 30 dollars for an output driver IC, which is more like 50 nowadays) The trick was, even if all THREE of the hard ones had the right parts for the easy ones, you take one part from each of the hard ones, and play it off like "I could of course fix those as well, but you'd have to replace that 30 dollar part I just pulled, plus the other part thats bad,which is probably going to be another 30 bucks, so its better to just keep those as parts." I recall one guy always wanted me to carefully solder back in the bad components into the donor monitors and reassemble them. He bought these horribly cheap imported monitors he always had a big stack of ones that had come back under the warranty period for his white box PCs) He would have me repair the easy ones. Some of them would be used to replace the warranty units (which is a perfectly acceptable practice, presumably wherever he was getting them, there was no manufacturers warranty). The ones that would clean up to look perfect, or that we could take parts from donors for, he would sell as new. (Which IS NOT a perfectly acceptable practice). He would ten take the ones I used as donors and carefully put back together and sell them to unsuspecting fools that were looking for a dead monitor and hoping they could fix it. (Good luck with that when there are 10 dead components. I guess if you figured out someone had been in it, you could guess that components with perfect shiny solder joints had been replaced,since most of the others were cold dull solder joints with barely any solder at all on them) One time he actually suggested that if I knew anyone who needed a PC I should send them his way. (The people I hate that much are on a very short list)

    Heres the schematic (The site elektrotanya BTW is one of the best sites I have ever found for repair manuals for almost ANYTHING. They have a lot of monitor, TV and laptop schematics amont others) https://elektrotanya.com/amstrad_pc14m28lr_pc14m39_y2_svga_monitor.pdf/download.html

    This one is very similar to the IBM. On the IBM monitor, there was a vref pin on the IC that was connected to the color lines through a 10K resistor. On the Amstrad it connects to the emitter of a transistor through a 10K resistor. There is also a capacitor in the Amstrad design, of similar value, but only rated at 10V, but nothing analogous to the 390 ohm resistor. I like the IBM better.

    If we look at the LM1203 in the IBM
    http://cdn.goldmine-elec.com/datasheet/A10692.pdf

    Figure 6 shows a simplified schematic of a video amplifier. The data sheet does not list a maximum input current, but DOES say the maximum voltage at any pin cannot be more than VCC. This makes sense, because if the voltage is greater than VCC then the collector base junctions of the input transistors start to conduct which could lead to bad things very fast. At 12V the input current into a amplifier pin would be 11.4mA through the 1000ohm resistor and 21.6mA through the 500ohm resistor for a total of 34mA. (assuming .6V drop across the base emitter junction of the input transistor,and the second transistor connected to the emitter of the input transistor) I've seen devices such as the LM1203 go out from time to time. My assumption was always that it took a static hit, because its connected to an outside line. All things considered there seems to be layer upon layer of protection that should keep it safe from anything but something like 50,000 volts from a static hit or EMP from a lighting strike near a long cable run. (I saw one like that one time. The modem in the PC connected to the phone lines and the monitor and video card that were connected via a 30' cable had melted down. They said lighting struck by the building.)


  • Thanks for sharing Michael. Interesting to see how real world CRT monitor implementations protected their inputs. Yes that beefier protection on the IBM one was probably nicer, but both should do the job for what a P2 could do to the directly attached VGA port.

    Looks like you had plenty of fun back then with your repairs. Those late 80s and 90s were such good times for mucking about with PCs (perhaps life in general) and I miss it now. All those frequent upgrades for your 2x gains and and getting it all to work together. Gone now. But now is a good time for hacking about with electronics stuff and microcontrollers etc so I guess that's something.
  • kwinnkwinn Posts: 8,697
    rogloh wrote: »
    ......

    Looks like you had plenty of fun back then with your repairs. Those late 80s and 90s were such good times for mucking about with PCs (perhaps life in general) and I miss it now. All those frequent upgrades for your 2x gains and and getting it all to work together. Gone now. But now is a good time for hacking about with electronics stuff and microcontrollers etc so I guess that's something.

    We fondly remember all the good stuff and forget most of the bad, except for the rare truly bad things. That's why we call them the good old days.
  • rogloh wrote: »
    Thanks for sharing Michael. Interesting to see how real world CRT monitor implementations protected their inputs. Yes that beefier protection on the IBM one was probably nicer, but both should do the job for what a P2 could do to the directly attached VGA port.

    Looks like you had plenty of fun back then with your repairs. Those late 80s and 90s were such good times for mucking about with PCs (perhaps life in general) and I miss it now. All those frequent upgrades for your 2x gains and and getting it all to work together. Gone now. But now is a good time for hacking about with electronics stuff and microcontrollers etc so I guess that's something.

    For a while there mucking about with hardware was dying out, then suddenly there was the "maker movement" and it came back with a vengeance.
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