Canon Speedlite 600EX II-RT Manual (en)

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Wireless Settings
To avoid interference with optical transmission wireless flash systems 
used by other photographers, you can change the transmission 
channel. Set the same channel for both the master unit and slave 
unit.
1
Display <C>.
 
On the master unit, press function 
button 4 to display <O>.
 
On the slave unit, press function 
button 4 to display <N>.
2
Set a transmission channel.
 
Press function button 1 <C>.
 
Turn <9> to select any channel 
between Ch.1 to 4, then press <8>.
Transmission Channel Setting
If the transmission channels of the master unit and slave unit are different, 
the slave unit will not fire. Set both to the same number.