Why not setup your EISCs to communicate by host name, and put the carts on the control subnet of the AV3 rather than in the main network. That way it's isolated to the room like Lincoln suggested, but using Ethernet rather than rs-232.
This presents you with 2 options- assuming both carts are identical, you can set the host names the same in each DMPS and use a single EISC to talk regardless of which is connected.
Or set each to a unique host name and rather than relays, you can use the online event for the EISC to tell which is connected.
- chris
