Client/Server and Master/Slave

Client and Server (TCP)

The terms Client and Server are used to describe Modbus TCP devices.
A Modbus Client established a connection (virtual circuit) on the TCP network to a Modbus Server. Through the established connection the Client requests information from the Server and the Server replies to the client with the requested information.
A typical industrial control application will consist of a network with multiple Servers (i.e. Instruments or PLC Inputs/Outputs) and one (maybe more) Clients (e.g. PLC CPU or SCADA system).
Both Client and Server are identified by a unique IP address.
TCP Network Diagram

Master and Slave (RTU)

The terms Master and Slave are used to describe Modbus RTU devices.
A Modbus Slave resides on a serial network (e.g. RS485) and listens for requests from a Modbus Master. If a request for information is received the Slave sends a reply (on the serial line) to the Modbus Server.
A typical industrial control application will consist of a serial network (multidrop) with multiple Slaves and at least one Master who issues a request for information to one Slave at a time.
RTU Network Diagram

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