I use two older, 580ex's as A & B slaves (shown below) and a third 580exII functions as disabled on-camera master.
Recently I have noticed 580EX "B" slave is underexposing a bit. When I swap the two 580EX's (A switched as B assignation), the "A" side is now underexposing. So I think this has something to do with the physical B unit and not an issue at the level of the master or settings.
I now see the B unit always displays "C. fn" on LCD whatever I do with the settings (see picture). The kicker is that both 580EX units are set at C.fn values of "1/0" when I check. There appears to be no way for me to get the B unit C.fn off on the display. To be fair, I really wouldn't even have noticed the display difference between the two 580EX units or cared had it not been for the underexposure issue, so can't rule out it had always displayed that way on this unit. But the exposure issue is recent. So I am not sure if I toggled something or if the unit is malfunctioning or if the C.fn display is normal.
1) Could the display difference be just due to variance based on when or where the 580 units were bought from? "A" is newer and was bought in Europe over a trip, while B is from the US...
2) Is there a way to reset the whole B unit? I am a tad worried if the C. fn funciton is somehow toggled "on" in B unit and is responsible for the exposure discrepency. I am not a speedlite whizz so I am hoping there is an easy answer.