It looks like Controller box and arm plate are connected with several power-wires and one or two twisted pairs (RS-485 or like this). Robot itself could be online controlled via several protocols: RTDE, MODBUS, URScript with direct socket connection (TCP/IP). Which of them is “the closest” to box-arm communication by response time and frequency metrics? Is it possible, that for example, MODBUS server on controller box is shared for external clients and arm-plate while RTDE is the abstraction on that?
The RTDE interface provides the best real-time capabilities.
A recommended approach would be to have a URP or URScript program running on the robot, and transmitting/receiving data externally from the RTDE interface.