Trigger Circuit Voltages are important to keep an eye on to prevent deep-frying your camera by using a flash with a trigger voltage way too high.
The trigger circuit voltages for EOS cameras were once published by Chuck Westfall in The Digital Journalist (March 2007 issue). In the same article he also wrote, that the trigger circuit voltages of future models will most likely be 250V, since they will most likely use an electronic shutter, which the trigger circuit depends on (also see this quote and its approval).
Models with mechanical shutter only had 6V circuits.
The trigger voltage of the x-contacts and of PC ports as far as available are identical.
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