Canon Speedlite 600EX II-RT Manual (en)

This section describes basic fully 
automatic wireless shooting when using 
a 600EX II-RT attached to the camera 
(master) and a 600EX II-RT set as a 
slave unit.
Set the master unit.
Set the 600EX II-RT attached to the 
camera as the master unit (p.63).
You can also use a device equipped 
with the radio transmission wireless 
master function as the master unit.
Set the slave unit.
Set the 600EX II-RT to be controlled 
wirelessly from the master as the 
slave unit (p.63).
You can also use other EX-series 
Speedlites that are equipped with a radio 
transmission wireless slave function.
Set the firing group to A, B, or C 
(p.73). The flash will not fire if it is set 
to D or E.
Check the channel and ID.
If the transmission channels and 
wireless radio IDs of the master unit 
and slave unit are different, set them 
to the same settings (p.64, 65).
a: Fully Automatic Wireless Flash Photography
Autoflash Shooting Using One Slave Unit
Firing group