What is true for all busses is also true for Modbus-TCP: the Black Channel principle separates data transfer mechanisms from the safety layer, i.e. leaves the actual Modbus routines untouched. Ethernet-enabled Modbus is not limited to TCP/IP communication, but may also utilize UDP/IP. openSAFETY exploits this option. Non-cyclical safety data is transferred via TCP/IP frames. Cyclical data, on the other hand, which is duplicated due to openSAFETY’s safeguard mechanisms anyway, is delivered via UDP/IP.
The openSAFETY over Modbus-TCP specification covers all details.