Canon Speedlite 600EX II-RT Manual (en)

Troubleshooting Guide
Remote release is not possible from a slave unit with cameras 
released up to 2011 or with EOS REBEL T6/1300D or EOS REBEL 
The master unit’s LCD panel illuminates or turns off according to the 
charge status of the slave units (firing groups). See “LCD Panel 
Illumination” on page 71.
Linked shooting
If you fire multiple Speedlites simultaneously 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 a self-timer to space out the timing of firing.
When used with a camera released up to 2011 or with EOS REBEL 
T6/1300D or EOS REBEL T5/1200D, the unit can be used only as 
“master camera unit”. The unit cannot be used as a “slave camera 
Cannot remote release from a slave unit.
The LCD panel illumination turns on and off.
Standard exposure is not obtained. /
Uneven exposure occurs.
The unit cannot be used as a slave camera unit.