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The PC Transporter provides composite out via a cable run to the internal video header of the Apple II. In order to do this, I assume it has to disable the sync and colorburst generators so the Apple doesn't interfere with its own signal. (Although it is possible they do something tricky to sync up their output with the signals generated by the Apple.)

I'm interested in doing the same so I can have a Raspberry Pi running Apple2pi installed internally using the built-in composite. I know I'll have to make some tweaks on the Pi side too, so it doesn't generate a composite signal until the Apple is booted to the apple2pi software, but unless I figure out how to disable composite on the Apple there's not much point in working that out.

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Partial answer:

Having looked at the manual, the only connection is a two-wire cable to video aux.

They also say that any composite monitor connected to the internal Apple video must be NTSC composite compatible.

As the Apple 6502 keeps running, I would assume they have the Apple display a black screen, and then sync up and overlay their own pixel outputs on the black level. After all, the manual says they have designed their own CGA implementation, so that wouldn't be much more effort.

I'm interested in doing the same so I can have a Raspberry Pi running Apple2pi installed internally using the built-in composite.

The alternative that I know 80-column cards used was to have a composite-in (connected to the Apple video output) and composite-out connector (connected to the monitor), and then switch between the Apple video signal and their own signal using anlog hardware.

So with some additional hardware connected to some GPIO pin on the RaspPi, you should be able to do that from a RaspPi as well.

Googling finds lots of discussion how to do that, for example with a MAX4649 chip and an Op-Amp, or this electronics.stackexchange question.

There are plenty of more ways, but I guess an implementation with a one or two chips and a number of resistors on a perfboard or similar shouldn't be too hard.

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  • Yeah, having something to let the pi control switching between the two sources is my fallback plan. Maybe I can put it on the adapter board I'm planning on making to attach the Pi to the Super Serial card's serial header. I imagine getting the Pi to sync up with the Apple's nonstandard sync simply isn't going to happen. It would be a fun project to make a small FPGA compose the Pi's output onto the Apple's signal, but I was hoping for a simpler solution. – Katie Jun 30 at 17:21
  • If I don't get a software solution for fully blanking the video soon I'll mark this as an answer. – Katie Jun 30 at 17:22
  • Keep in mind that if you use the internal video header, you not only have to sync, you also have to blank the Apple video output, because you cannot "overwrite" the current pixel value on the internal header. I don't know what kind of combined system you envision for RaspPi + Apple, but I assume you don't want to use the Apple as "I/O processor" like the PC Transporter does. – dirkt Jun 30 at 20:47
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The PC Transporter provides composite out via a cable run to the internal video header of the Apple II.

This could be a missconception. The Apple II does not have a Video-In. The 4 pin connector right of Slot 7 provides the NTSC signal and is in general meant to augment it, like with a PAL converter for European TV.

Your intention is simply to use the Apple's RCA connector, so the user doesn't have to switch connectors. In general I would recomend to route the Apples signal from the 4 pin connector over to your card and have an analogue switcher between the Apple II's signal and whatever it produces, to finally be output on a new connector. Much the same way most 80 columns cards for the Apple II+ work. This is safe and carries no chance to damage the II.

Now if you want to do it the PC-Transporter way, then the only clean way is with the Apple IIe and closing the X3 pad. This will connect the AN3 output to the video logic, disabling the signal, thus closing the output transistor. Now an external applied signal can be applied and will show up at the video jack. See here for a short description.

The Apple II/+ isn't prepared for this, so you need to throw a wire across two chips.

After doing the modification AN3 can switch video on and off.

Another, less elegant way would be to drive an all black picture and overwrite the Apple II signal.

This will of course need previous synchronisation (and resynchronisation) using the same pin. Not sure if the is as easy possible as you seam to expect.

Either way will require careful design to not damage the II's output circuitry. Also, neither solution will work with the IIgs.

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  • The PC Transporter manual connects a cable from its composite out to the internal video header. From the schematic, there are only a few resistors between that and the composite out. So even though it isn't intended as an input, it is possible to drive the output through it. The PC Transporter manual also states that when hooked up that way you can use the built-in composite out to view Apple II or Transporter video. – Katie Jun 30 at 17:10
  • @Katie So? I haven't said it can't be done, but don't you think there is a reason that next to no other video generating card has used it that way? It simply isn't as trivial. With software blackened screen (or hardware modification), the Apple II will still drive a video output. This can't simply be overridden. Any signal to 'replace' it needs to be synchronized to existing (black) image. The manual only states how this looks to a user, not what they done to archive it. Look at the schematics and it'll be quite clear what's possible and why it may damage the video out circuitry. – Raffzahn Jul 1 at 0:05

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