Canon EOS 5DS Instruction Manual page 246

D Flash Photography
If the camera is used with a flash unit or flash accessory dedicated to
another camera brand, the camera may not operate properly and
malfunction may result.
Do not connect to the camera's PC terminal any flash unit with an output
voltage of 250 V or more.
Do not attach a high-voltage flash unit on the camera's hot shoe. It may
not fire.
A flash unit attached to the camera's hot shoe and a flash unit connected to
the PC terminal can both be used at the same time.
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