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Thread: Canon cameras, flashes and how they talk to each other.

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    Canon cameras, flashes and how they talk to each other.

    I have a canon camera and a canon flash.

    Somewhere I have a picture of the hot seat and what each contact does.

    But are these contacts simple signals, or is there data sent between the two things?

    So: Flash read, shutter release, TTL signal...... Are they just "ON/OFF" signals or is it more complicated?
    +===========================================+
    Canon EOS 550D 18-135 (IS) lens 90-300 lens
    +===========================================+

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    Being everything modern now a days is micro processed, my GUESS is code, Focal Length, Shutter Duration, and so on, Which curtain to fire on.

    Why?

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    I'm kinda bored.

    I was going to buy a "trigger trap" thing which is a computer controlled camera system to do time lapse, sound, light, and so on pictures.
    Alas they are not available now and a new model is "coming".

    I am in the process of building an interface to tell the camera to "focus" and "shoot" - that is easy.

    I've read that it is easy to detect the camera taking a picture. All you do is monitor the resistance between the main (big) pin and the chassis of the camera. Canon allow up to 5 v (dc) and when the shutter fires, those two pins are shorted.

    But as I am also playing with second curtain firing of the flash I am interested how the camera tells the flash it has enough light.

    I'm guessing with first curtain, it "simply" does the second curtain and stops the light.

    I haven't got too much into it yet. Building the interface has been a bit annoying to say. Getting the time and patients to do it. Now it is done.

    So I was wondering what else goes on between them.

    Is there a simple "Enough light" signal the camera sends the flash and so the flash stops "flashing"?

    I may have to buy one of these leads which allow remote flashes and get into it and get the leads out and see what is going on in there, signals etc.

    But I was asking to see if anyone has already got the data.

    Mr Google isn't being too helpful as yet.

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    From Memory, under "Flash - Strobist" on AP, there was a link to a very helpful Flash site, with indepth tutorials and information on all thing Flash. I found it very helpful when I first started playing with off camera flash.

    May be worth a search, form memory it went into the science behind E-TTL and how it works/interfaces with the camera body.

    Have fun,

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    Um, I'm down to page 9 and either have missed it or I'm stupid.

    A wee bit more info would be nice.

    Thanks.

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    If I recall correctly, the eTTL system uses a simple SPI based protocol. Meaning: no, the pins do not carry simple "on/off" signals. Google on "canon eTTL protocol" or something similar should provide the info you're after.

    Anyway. SPI is a pure master/slave, clock synchronized protocol that uses a clock signal (SCLK), a slave select signal (/SS) and 2 data pins (MISO and MOSI) (and GND of course). These signals usually are defined to use TTL levels in a push-pull configuration (so take care when experimenting, it's pretty easy to fry them).

    There are four different modes the SPI data sync can use, defined by clock polarity and data phase synchronization. Data is sent MSB first and it usually (but not necessarily) is 8-bit oriented (I've seen 12- and 16-bit implementations too so YMMV). Clock frequencies usually are no sludge (a couple of MHz is not uncommon). The defacto standard (there's no ISO standard, but everyone looks at the original Motorola / Freescale docs) does not define max. clockspeeds, those are usually dictated by the slave's hardware and the master's capabilities. Clock speeds usually don't set a minimum speed (though fall- and rise time limits usually are defined).

    I guess what I'm trying to say is: SPI is not meant to be tampered with if you do not have at least some basic digital electronics skills and knowledge of chip-to-chip protocols.

    On a software level, the camera probably sends requested output power, zoom level and some other information. There is no synchronization on this level on when to "stop flashing".

    Having said that, there's also a "dumb mode" available. The outside of the hotshoe contains a spring contact providing GND. In the center, there's a bit pin. If you short-circuit those on the flash, it will fire. If you put the flash in manual mode, you can trigger it here.

    PS: Roosta may have referred to a flash tutorial I wrong a long time ago. This one => http://www.ausphotography.net.au/for...lash-equipment

    Edit #2: It seems the images are missing from that tutorial. Dunno why, you may have to ask Rick about that.
    Last edited by jev; 11-03-2014 at 7:37pm.
    Ciao, Joost

    All feedback is highly appreciated!

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