TTL flash. Manual slave ones won't flash over speed sync.

[lamacchiacosta]

Hello,

I would like to understand a bit more deeply about using flashlights.
I've got a Godox TT685 on my camera or triggered with a Xpro. I also have a couple of manual Yongnuo YN660 I'd like to use not to let them gather dust on the shelf. In order to do that, I set them as slave, so they will flash when the sensor will trigger them.

When I get over the maximum speed sync of 1/250 on my X-T3, the Yongnuo won't flash, either in S1 and S2 (the Yongnuo), both in TTL and manual modes (the Godox).
Could anyone explain why?
Many thanks in advance.

[itchy shutter finger]

Does everything work at 1/250 or slower? Does the Godox work faster than 1/250?

While I have none of the equipment you describe, I wonder if a firmware in the Godox or Yongnuo prevents operation above sync speed. 

[lamacchiacosta]

Hi,
yes all the flashlights work over the ss of 1/250. When I use them independently (not in Slave mode) they do what they are made for.
Bot units have the latest firmware, I don't think they are prevented to flash when over ss.