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22Mar/213

Power Mac 6100/66 DOS Compatible – MIDI Out

After building the 'hydra cable' for this DOS Compatible machine, I quickly found out that the Joystick port could not be used for MIDI input/output. There were numerous hints online, and they all resulted to the fact that we'd need to solder a pin on the Vibra16 IC to get midi to the port. Here's the module removed from the DOS card. The CT2501 IC nearly takes up the entire board.

DSC02950 DSC02956 DSC02958

I was able to re-wire all of the below as I created my own hydra cable. I wouldn't suggest trying to modify an off-the-shelf cable as they're a sealed unit and have many pins incorrectly wired at the joystick-port-end.

Determining the correct pin

I'd asked online for help from someone who'd already worked out the solution, but hadn't received a quick answer. After rummaging through my junk boxes, I realised I didn't have a card with the correct CT2501 chipset, so I asked a few friends also. No one had the exact model required, so I considered purchasing one on eBay. They were averaging about one-hundred-dwollar shipped, so I decided to try and find high resolution photos online instead.

It turns out that dosdays has a fantastic library of information on the SB16. Included are high-enough-res photos of the rear of the board that allowed me to easily trace pin 12 (MIDI OUT) back to the CT2501 IC. Pin 15 (MIDI IN) wasn't so easy as it disappears under the 74-series IC in the bottom left of the top-side of the board. Fortunately, my main goal was to just get DOOM II throwing MIDI out to my SC-88.

sb16-bottom sb16-top ct2500-close-up

After sitting in Paint.NET for a while, tracing traces, I came to the conclusion that MIDI OUT was Pin 30 of CT2501. In the pictures above, I've traced this in RED on the underside-shot of the board and YELLOW on the zoomed in CPU picture. MIDI IN is traced in BLUE on the underside-shot, but then disappears on the top half under the IC. Images from the web don't provide enough detail to allow me to continue the search! I was happy with just MIDI OUT, so I then started the quick hack-job to solder on a test wire...

ct2501-close-up ct2501-close-up-2 ct2501-close-up-soldered

In no time at all, DOOM II was outputting its soundtrack into my headphones.

DSC02980

Usually I'd pipe the audio-out from the SC-88 back into the soundcard of the computer that it was connected to, but that's not an option here with the Power Mac 6100 as it only has Microphone In! Oh wait, can I also wire-up Line-In for the DOS Card? I don't think that'd be as straight-forward as I'm sure there'd be a DAC and other components on the path.

External Connections

The port at the back only needed to be one wire, so I initially used an RCA socket.

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I then realised I had no spare RCA plugs on-hand, so I switched the socket to a 3.5mm mono audio jack. This looked nicer anyway.

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From here, a headphone cable was wired through with one of the channels running to pin 12 on the joystick port. This was then plugged into the SC-88 and DOOM II WAS PLAYED AT FULL VOLUME!

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  1. I’ve recently purchased a LC 630CD DOS Compatible, unlike your system it doesn’t have a y-cable, instead there is an internal board with a joystick port attatched. I’m wondering two things: 1. do the pins for my joystick port match yours, I guess I’d have test with a volt meter on pin 12 to see. 2. If everything is essentially the same, can you provide a bit more detail of how you went from the wire soldered on to pin 12 of the vibra chip to a connection you could hook up to your midi device.

    • Can you provide a bit more info on the final steps, e.g. how did you go from having a wire coming from the vibra chip to using a 3.5mm mono audio jack to having a connector that connects to your SC-88 midi connection?

    • Hi Steve,
      In the ‘Hydra’ Cable, the MIDI-Out pin is hard-wired and no good for actual MIDI devices. Therefore, when I soldered up my own DB-15 port, I wired the pin off the Vibra chip through to the MIDI out port on the joystick port.
      From there, you connect a standard DB-15 to DIN-5 MIDI cable where MIDI-OUT is wired through to the ‘Out’ cable which is then plugged into the MIDI sequencer.

      I’m currently on holidays, but can research your board when I get home. We can then work out what to do and if there’s an option for yours. If you already have a joystick port on-board, then the re-wiring will be very difficult.. but it could be done if we get a male+female DB-15 and skip the on-board wire, providing the MIDI signal alternately.

      I’ll send you an email now and follow up in a week or two.

      Steven.


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