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Monitor identification over the built-in video port on Beige G3


Well-known member
TL;DR Did someone who owns a Beige G3 managed to connect a DDC capable monitor to the built-in video card?

Long story: I'm currently writing an emulator for the onboard video (ATI Rage Pro) meant to be integrated in the new PowerPC Macintosh emulator called DingusPPC.

While analyzing the way Macintosh SW communicates with the HW I noticed the following strange behavior - the built-in video card seems to support two monitor identification schemes:

  1. the legacy one uses three monitor sense pins to obtain extended monitor code as described in the Technical Note HW30
  2. the other one supports the Display Data Channel (DDC) standard

Monitor identification works over so called gossamer monitor sense. There is a bit in the I/O controller (Heathrow ASIC) to enable/disable this sense.

ATI drivers enable it before accessing three predefined GPIO pins of the ATI chip.

Now comes the actual puzzle: (WARNING: possibly too much technical details!)

It looks like the SENSE 2 and SENSE 1 lines are shared with SDA and SCL lines used to implement the DDC I2C protocol.

ATI drivers try DDC first and switch to the legacy Apple monitor sensing scheme if no EDID data was received within a predefined time period.

There should be theoretically a switch between these those schemes.

Unfortunately, I could not find any reasonable switch except the following strange command sequence:

Before trying DDC, the driver sets the direction of all three GPIO pins to "input" and then writes "1" to these pins!

It doesn't make any sense to me - writing anything to GPIO pins configured for input will be supposedly ignored by the circuit.

There is a possibility that Apple used some tricky circuit on the motherboard. Unfortunately, no information regarding that monitor sense is publicly available. Figuring this out requires a working Beige Mac with a DDC capable monitor. I have neither of them.

Another crazy idea is to connect a logic analyzer to the sense pins of the video port and record all electrical signals arriving there followed by a detailed analysis. I'm afraid I can't do it at home.

I therefore decided to ask here. If someone has a DDC monitor working over the built-in video port, this would be a clear evidence for that port actually supporting DDC.

I assume that monitors Apple originally shipped with Beige G3 aren't DDC capable.

There is a HD15-to-DB15 adapter manufactured by Griffin that offers the Plug&Play option as well as eight switches to select the desired monitor resolution.

That Plug&Play mode sounds like it would use DDC but I'm not sure if it works with Beige G3.

P.S. it should be definitely the built-in video port, not a PCI video card. IIRC, external ATI cards don't support Apple monitor identification scheme...

(Sorry for the long post...)

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