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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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    \$\begingroup\$ To what end do you wish to know? \$\endgroup\$
    – OnBreak.
    Commented Mar 30, 2019 at 18:27
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    \$\begingroup\$ 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. \$\endgroup\$
    – rackandboneman
    Commented Mar 30, 2019 at 18:32
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    \$\begingroup\$ kwga.com/blog/canon-ettl-protocol-investigation ... this makes it seem like at least the modern (DSLR relevant) protocol isn't officially documented. \$\endgroup\$
    – rackandboneman
    Commented Mar 30, 2019 at 18:35
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    \$\begingroup\$ BTW, the quench pin is all about film TTL protocol. \$\endgroup\$
    – rackandboneman
    Commented Mar 30, 2019 at 18:37
  • \$\begingroup\$ I can help with the trigger voltages on canons. For a modern dslr beyond about t1i, it is a low voltage. 5 volts before it’s grounded. Older canons have a much higher voltage that requires a tiny relay or FET. \$\endgroup\$
    – Mark Robin
    Commented May 1, 2022 at 14:50

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Canon considers their flash hotshoe pinout to be proprietary information and does not publicly post it anywhere. Companies like Godox make products by spending a lot of time and engineering expertise to try to reverse-engineer this type of communication protocol.

The only known signal is the one specified by the ISO standards for hotshoes/flash feet: the big center contact is sync, the rails are ground, and shorting the two is the signal to fire the flash.

And at any rate, the five contact/pin communication (and ISO compatibility) may become a thing of the past now that Canon has moved to their new multi-function hotshoe with the R3 and later bodies, which have contacts on the front edge of the foot plate/hotshoe, like Sony's multi-interface hotshoes/flash feet. And Canon's latest flash, the EL-5 has no pins on the bottom of its foot:

bottom of the EL-5 foot

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  • \$\begingroup\$ I hope someone reverse engineer the new multi-function hotshoe, i need one more input 3.5 mm Jack on my R5c \$\endgroup\$
    – baaron
    Commented Jan 24 at 4:35
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    \$\begingroup\$ For those wondering the Sony multi-function hotshoe has been reverse engineered here: mhohner.de/sony-minolta/flashcomp_techref.php I don't know if this is the same as canons multi-function hotshoe but they look compatible so would be silly plugging one into the other would break the accessory/camera \$\endgroup\$
    – user3797758
    Commented May 8 at 23:02

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