90EX fires when used as Master (it should not fire)

Hi, I have no experience of using the 90Ex - it may well not offer HSS at all, the built-in pop-up flash Commanders do not allow HSS across a Canon Wireless network.

5xxEx master units (& the ST-E2) do offer HSS, but I only mentioned HSS in response to LincolnB's interesting thought.

It's possible the 90Ex is limited in other ways too. Under Canon Wireless Light/IR type using a 5xxEx as master, there are 2 ways of using slaves in M. 1) First is to initially leave the slaves showing ETTL on their screens, then switch the master to Manual (or Flash Mode to Manual on a cam's flash menu screen), then after a pre-flash, the slaves will obey M flash power fraction by Group. And the M, newly appearing on their screens will not blink at you. The Ex slaves remain both controllable & triggerable from the master device. 2) Or, Second, you can put Ex slaves into M using their own buttons/controls (not remotely) in which case their M will blink on their screens at you. Power fractions must be then set on the slaves themselves. The blinking M reminds you that while they remain triggerable remotely, they are no longer controlable/adjustable remotely! I'm not sure how much of this is offered by a 90Ex. Remember any remote control/adjustment instructions (whether ETTL or M) are only enacted by a pre-flash.

I'll try to read an on-line 90Ex manual ....

Edit: Yep, the 90Ex can offer HSS and it can respond if the attached camera has its Flash Mode line set to Manual instead of ETTL. So basically anything allowed within the cam's Flash menu screens will apply. Obviously line of light - the pre-flash light path, whether direct or bounced - issues still apply and there is no focus assist while controlling wirelessly, but otherwise it looks OK for the size & Wt.