Canon Speedlite 600EX-RT Mode D'emploi page 112

Troubleshooting Guide
<k> is displayed.
Set the shutter speed 1 stop slower than the flash sync speed (p.51).
Cannot release from a slave unit.
When an EOS camera which was released up to 2011, has an N3 type
remote control terminal and is compatible with E-TTL II/E-TTL autoflash
is used to perform remote release from a slave unit or when it has been
set as the slave unit during linked shooting, the "Release Cable SR-N3"
(sold separately) is necessary (p.68, 70, 104).
Linked Shooting
Uneven exposure occurs./The standard exposure is not obtained.
If you fire multiple flash units at the same time during linked shooting,
the appropriate exposure may not be obtained or uneven exposure
may result. It is recommended to set only one Speedlite to fire or to use
self-timer to space out the timing of the flashes.
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