I haven't found any thorough documentation about exactly what each pin does and the signalling between a camera (say a T6i) and a flash (say speedlite 600ex-rt ii) looks like. What voltage levels, what is sent from the camera to the flash and vice versa, sequence of communication, etc.

I've gathered at least that shorting Trigger to Ground tells the flash to fire (I assume at max and/or at the certain settings and immediately?).

The best image I've found so far is from here. Flash pinouts

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    To what end do you wish to know?
    – OnBreak.
    Mar 30 '19 at 18:27
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    Also, mind that the signalling used will be quite different when comparing an early film camera (eg an A-1), a modern film camera (eg an EOS-V), and a DSLR or DSLM... even if the hotshoe looks the same. Mar 30 '19 at 18:32
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    kwga.com/blog/canon-ettl-protocol-investigation ... this makes it seem like at least the modern (DSLR relevant) protocol isn't officially documented. Mar 30 '19 at 18:35
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    BTW, the quench pin is all about film TTL protocol. Mar 30 '19 at 18:37

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