1. Protocol refers to the rules of operation followed by peer entities. The Protocol defines the format of PDU’s (Protocol Data Units) and their rules of operation.
  2. Modbus TCP/IP follows the Client-Server model. Modbus masters are referred to as clients, while Modbus slaves are servers.
  3. A client (or master) is any network device that sends data requests to servers (or slaves).
  4. A server (or slave) is any program that awaits data requests to be sent to it. Servers do not initiate contact with clients, but only respond to them. Some devices operate as both clients and servers.
  5. A port is an address that is used locally at the transport layer (on one node) and identifies the source and destination of the packet inside the same node. Port numbers are divided between wellknown port numbers (0-1023), registered user port numbers (1024- 49151), and private/dynamic port numbers (49152-65535). Ports allow TCP/IP to multiplex and demultiplex a sequence of IP datagrams that need to go to many different (simultaneous) application processes. Modbus TCP/IP uses well-known port 502 to listen and receive Modbus messages over Ethernet. 



Source:

https://www.prosoft-technology.com/kb/assets/intro_modbustcp.pdf

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