Modbus communication over Ethernet TCP/IP is of client-server type. This protection relay operates as a Modbus server.
The Modbus TCP/IP connection is established when the Modbus client opens a TCP socket connection to the Modbus server. The socket port 502 on the TCP/IP stack is reserved for Modbus. If the connection request is accepted by the server, the client can start communicating with the server unit.
Protection relays can usually accept several simultaneous Modbus TCP/IP client connections even though the number of connections is limited. The protection relay can be configured to only accept socket connection requests from known client IP addresses.