When does X-SYNC trigger the flash?

<p>When X-SYNC pin goes LOW(GND) or HIGH(VCC)? At one point in my life I knew this information readily but I can not seem to remember in high confidence. IIRC: X-SYNC fires when it goes LOW(GND).</p>

<p>Thanks...</p>

<p>X-sync goes short the center contact to frame ground. It is not a signal, it is a closed switch.</p>

<p> </p>

<p>As Bob said it's a mechanical switch on the shutter that short circuit to ground or to be correct it was once upon a time. Nowadays it's actually an <a href="http://en.wikipedia.org/wiki/Open_collector">open collector signal</a>, meaning some type of semiconductor in the camera will pull the X-sync to ground when it's time to fire. For all practical purposes it still works like a mechanical switch.</p>

<p>The flash or whatever device that sits in the hot shoe or pc sync will provide a little voltage and some current to the pin and sense when the camera pulls it low.</p>

<p>On Nikon it's a triac that pulls the signal low and the voltage provided by a Nikon flash is around 3V. If you measure the pin with the flash connected it will go from 3V down to GND when the camera fires the flash.</p>

<p> </p>

<p>If you ever forget, just remember that the protocol for X-sync dates from the days before cameras had batteries in them. The PC contact has two wires only. A purely mechanical camera can't easily put voltage on the wires, but it can easily short them together via metal contacts in the shutter. So that's why the protocol is that the camera shorts the two wires together, and the flash is responsible for providing a bit of voltage. The voltage and polarity provided by the flash isn't standardized.</p>

<p>Most old flashes will work on modern cameras and most old cameras will trigger modern flashes. There are exceptions for some old flashes which use high voltages that are incompatible with the semiconductors in some modern cameras, so beware. But this compatibility with old cameras is why the protocol is the way it is. Besides, it's not broken, so there hasn't been a need to fix it.</p>

<p>And I thought he was asking as for example the difference between X and M sync, or 'second' :-)</p>