[quote user="Biser Gatchev-XID"]
I'm not an expert but maybe you should toggle a GPIO at the end of the ON command and measure delay between the two fronts.
[/quote]
This is exacty what I meant.
Let's say you have a Slave IO device. All the outputs are at 0V.
At T=0, the ECAT Master sends a command to turn on one of the outputs.
At T=(x)usecs, the slave output goes to 24V.
The latency is x microsecs.