how to start a gated counter with a digital trigger (6602)

[X.]

Hi,

I want to start counting events (photons arrival time on a photodiode)
with a 6602 board, and trigger the beginning of this buffered counting
task with a digital signal that is output by a 6611E board (I use this
trigger to start an anolog output and analog input on this same board).
The point is that I need a perfect synchronization between all
acquisitions and I can't figure out how to start a counter with a
trigger. It looks like a TIO board would have allowed me to have a
secondary input to do that, but with the 6602, it seems that I have to
rely on a software trigger.
I am a bit confused and any hint or comment would be appreciated.

Thanks in advance.

--
Xavier