packpe89 wrote in post #18768342
... My other flash is a 550EX. I like to [Shoot] off camera flash and have used my 550 to master the 420 on my 5D and the on-board flash to master the 550 and 420 on the 7D.
Can I still use the two flash setup with the equipment I'm getting?
Yes, but not over radio. The TT685-C can be either master or slave in the Canon "smart" optical system the Canon EX speedlights use. So you can use it the way you used the 420EX as an optical slave to either the 550EX or the 7D's pop-up.
One thing I like about the Godox system is shooting RF for range (Like for tele outside portraits, IR does not work well). How can I use the 550EX with RF? Do I also need to get a X1T-C to attached the 550 to? If so, is everything still E-ttl?
You'd need an X1R-C receiver, but the Godox radio triggers basically use the flash/camera menu communication signals to control the flashes, so it really only works fully with flashes that you can control directly through the camera menu (i.e., 580EXII, 430EXII and later models). The 550EX is likely to be only partially compatible, and (if they're like the Yongnuo YN-622 triggers) may only work in TTL.
It might be better to simply get a second TT685-C if you want to go the radio triggering route. It's more expensive, but will be fully compatible, and a built-in trigger is much more convenient/stable in setups than an add-on receiver. And, since the TT685C has a built-in transceiver, you can also use the TT685-C as both an on-camera flash and a radio transmitter for the other TT685C off-camera.