Modbus is an industrial protocol standard that has been in use for many years. Modbus ASCII and Modbus RTU are relatively simple serial protocols that use EIA-232 or EIA-485 to transmit data packets. The protocol defines function codes and the encoding scheme for transferring data as either single points (1-bit, coils) or as 16-bit data registers. This basic data packet is then encapsulated according to the protocol specifications for Modbus ASCII, RTU, or TCP.

The TCP version of Modbus follows the OSI Network Reference Model; however the serial implementations of Modbus do not. Modbus/TCP defines the presentation and application layers in the OSI model.

Modbus protocol is defined as a master/slave protocol, meaning a device operating as a master will poll one or more devices operating as a slave. This means a slave device cannot volunteer information; it must wait to be asked for it. The master will write data to a slave device’s registers, and read data from a slave device’s registers. A register address or register reference is always in the context of the slave’s registers.

Modbus/TCP makes the definition of master and slave less obvious because Ethernet allows peer to peer communication. The definition of client and server are better known entities in Ethernet based networking. In this context, the slave becomes the server and the master becomes the client. There can be more than one client obtaining data from a server. In Modbus terms, this means there can be multiple masters as well as multiple slaves. Rather than defining master and slave on a physical device by device basis, it now becomes the system designer’s responsibility to create logical associations between master and slave functionality.