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cga to scart adapter options

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    cga to scart adapter options

    I have seen a lot of different schematics for cga ttl to - analog rgb scart adapters.

    Found that this one is recommended:
    http://static.elitesecurity.org/uplo...TO%20SCART.jpg

    I see that pin 7 on the cga port is marked as 5+ but on pinouts of the port says (reserved) or (ttl video).
    Somebody knows more about this adapter, or that is is safe to use this schematic ?.

    Thanks!.

    #2
    The standard is for pin 7 on the CGA to be not connected, however some cards are dual CGA and MGA. On those cards pin 7 is the mono video signal.
    The is enough resistance in the schematic to not harm the video output of a mono card and a CGA that has no connection on pin 7 wont be harmed either.
    The lack of 5v being output by the video card means the circuit wont work. You will need to get a non switchable CGA video card that has no connection on pin 7 and connect it to a 5V source on the card.

    Comment


      #3
      That adapter will produce an incorrect color for #6 (brown); it will show up as light yellow. Otherwise it seems ok.

      Why is #6 brown? Answer.
      Offering a bounty for:
      - A working Sanyo MBC-775 or Logabax 1600
      - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

      Comment


        #4
        Thanks, for the reply's guys also joining together the sync signals using resistors is not an AC short ?

        Comment


          #5
          It has resistors, they are protected and attenuated.

          The transistor is just converting the signals to negative composed sync, but it won't be a serrated vsync. But the scart 2 hdmi doesn't care about it.

          It might work but just using a XNOR gate between H and V and adding a 470 ohm resistor at the output.

          The brown color could be fixed by adding decoders. If we had a fast EPROM (10ns) that would be simple, but they are damn slow. Maybe a GAL ?

          Comment


            #6
            See here. I have named the three different schematics variants "1 to 3" (this one is variant 1), there is even a fourth variant. There you can read my explanation for your pin 7 question.
            Note that the Green signal should be connected to SCART pin 11 instead of 4, this was an error in this diagram.

            I can say that I have built variant 3 and it works perfectly (without brown-fix however, but I don't really need that), see my pictures over there.

            Comment


              #7
              Originally posted by RobertK View Post
              (without brown-fix however, but I don't really need that)
              Why don't you need the brown fix?
              Offering a bounty for:
              - A working Sanyo MBC-775 or Logabax 1600
              - Music Construction Set, IBM Music Feature edition (has red sticker on front stating IBM Music Feature)

              Comment


                #8
                Originally posted by Trixter View Post
                Why don't you need the brown fix?
                Because most of the games I play use either one of the two standard CGA palettes, or cyan, red & white (which was also quite popular among game developers and good-looking IMHO). None of these are affected by the brown problem.

                But there is actually one CGA game where I would like to have the correct brown color displayed: Defender of the Crown (CGA). It still looks good without the brown fix, but of course it's not the real thing.

                Ok, Paku Paku also uses the brown color, but I don't mind seeing dark yellow instead of brown, the game is still great anyway!

                Comment


                  #9
                  Originally posted by RobertK View Post
                  Because most of the games I play use either one of the two standard CGA palettes, or cyan, red & white (which was also quite popular among game developers and good-looking IMHO). None of these are affected by the brown problem.

                  But there is actually one CGA game where I would like to have the correct brown color displayed: Defender of the Crown (CGA). It still looks good without the brown fix, but of course it's not the real thing.

                  Ok, Paku Paku also uses the brown color, but I don't mind seeing dark yellow instead of brown, the game is still great anyway!
                  Thanks for the link, i don't whant care for the brown color going to build one you provided in the shematic.
                  the only problem i see is that i have an original brown cga card from ibm, i don't think theres a signal on pin 7 to feed power to the transistor.
                  Maybe i have to add a separated power supply for it a cellphone one that provide 5v?

                  Comment


                    #10
                    Originally posted by soviet9922 View Post
                    ... i don't think theres a signal on pin 7 to feed power to the transistor.
                    Maybe i have to add a separated power supply for it a cellphone one that provide 5v?
                    You should be able get 5v from the ISA Bus pin B3. See Here Preferably, trace to a point on the card that can easily be soldered to and run a wire from there to the pin 7 on the connector as long as it is NC. If necessary, you can solder to the finger of the edge connector, but be careful not to let the solder get to far down on the finger so that it doesn't get into the ISA connector when the card is plugged in. You may need to cut a trace if pin 7 is connected. I don't have the schematic in front of me for the IBM CGA card but it should be easy enough to check visually or with a meter.

                    I believe this is what David_M indicated when he said "You will need to get a non switchable CGA video card that has no connection on pin 7 and connect it to a 5V source on the card." in post #2.
                    Last edited by ibmapc; October 15, 2017, 11:17 AM.

                    Comment


                      #11
                      Originally posted by ibmapc View Post
                      You should be able get 5v from the ISA Bus pin B3. See Here Preferably, trace to a point on the card that can easily be soldered to and run a wire from there to the pin 7 on the connector as long as it is NC. If necessary, you can solder to the finger of the edge connector, but be careful not to let the solder get to far down on the finger so that it doesn't get into the ISA connector when the card is plugged in. You may need to cut a trace if pin 7 is connected. I don't have the schematic in front of me for the IBM CGA card but it should be easy enough to check visually or with a meter.

                      I believe this is what David_M indicated when he said "You will need to get a non switchable CGA video card that has no connection on pin 7 and connect it to a 5V source on the card." in post #2.
                      Wouldn't be easier to use any wall wart psu ? Get an old cellphone charger (the ones before the USB) that does 5V and voila or buy these universal psu at any electronics shop.

                      Comment


                        #12
                        Originally posted by Retro Canada View Post
                        Wouldn't be easier to use any wall wart psu ? Get an old cellphone charger (the ones before the USB) that does 5V and voila or buy these universal psu at any electronics shop.
                        Might be "easier", but not as tidy. He would have another cord to manage and use another outlet on his power strip. Plus, he would need to purchase the "wall wart". So, in My mind, not easier after all.

                        Comment


                          #13
                          Hi soviet9922,
                          I would recommend to forget about the "pin 7" solution and use an external power supply. That of course means one more cable hanging from your adapter, but it has the advantage that your historic hardware will be kept unmodified. I simply connected a Micro-USB plug cable (so that I can plug in any cellphone charger), check your local electronic store for such a ready-made cable. I used item number 1414354 - 62 from Conrad which is currently sold out, but here is another such cable.

                          Comment


                            #14
                            Thanks for the ideas, guys i thing im going to use the external psu option so i can change the adapter to other systems easy.

                            Comment


                              #15
                              You will need to drive 12v to the pin 8 to activate the 4:3 aspect ratio as well 3V to pin 16 for the RGB blanking, otherwise it will take the composite input...

                              If you use the Scart 2 HMDI you can override the aspect ration on it using the OSD menu...

                              Comment

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