In fact, my initial thought is to transmit data at a rate of 200Hz from target 2 to target 1, even though I will likely log data at 100Hz at target 1. Logging rates are not yet fully defined, hence the reason why I leave myself some buffer. The data transmitted is solely for logging purposes, not control. However, skipping a few data points may make data interpretation trickier.
With regards to the protocol that both sides can speak, that is precisely what I'm looking for. If there already is a usable, well integrated solution for that I'd like to find out what it is. I'm trying to avoid low-level protocol integration if possible.
Of course, RM would be my pick, but I have to balance cost and need. If I get the same result through a well integrated ethernet solution, I'll go with it. However, if I need to spend a week integrating Ethernet data sharing between target 2 and target 1, then my time's certainly worth an additional RM module.
Regarding EtherCAT, I'll be using that technology on the load control side of my system. I'll have a master PXI with a few 9145 slaves, one for each of my loading station, where I'll be performing PID control at the FPGA level.