Canon EOS 1D Mark II N Instruction Manual page 114

Flash Photography
Non-Canon Flash Units
Sync Speed
The EOS-1D Mark II N can synchronize with compact, non-Canon flash
units at 1/250 sec. or slower shutter speeds. With large studio flash, the
sync speed is 1/125 sec. or slower. Be sure to test the flash to see if it
synchronizes properly with the camera.
PC Terminal
The camera's PC terminal is provided for flash units having a sync
cord. The PC terminal is threaded to prevent inadvertent
disconnection.
The camera's PC terminal has no polarity so you can connect any
sync cord regardless of its polarity.
If the camera is used with a flash unit (with dedicated flash contacts) or
flash accessory dedicated to another camera brand, the camera may not
operate properly and camera malfunction may result. Also, do not
connect to the camera's PC terminal any flash unit requiring 250 V or
higher voltage.
Do not attach a high voltage flash unit on the camera's hot shoe. It might
not work.
A Speedlite attached to the camera's hot shoe and a flash unit connected to
the PC terminal can be used at the same time.
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