Modbus Protocol - Modbus RTU, Modbus TCP, Modbus ASCII, Modbus Plus

Modbus is an application layer messaging protocol that provides client/server communication between devices connected on different types of buses and networks. It is available as a serial protocol - Modbus RTU or Modbus ASCII, an Ethernet protocol - Modbus TCP, or a fieldbus protocol - Modbus Plus. Also certain manufacturers have modified the standard Modbus protocol to meet their specific needs such as Modbus Daniels, Modbus Omniflow and Modbus TekAir. Modbus has been the industry’s defacto standard since 1979 when Modicon introduced the protocol in its PLCs. Modbus continues to enable millions of devices worldwide to communicate and it continues to grow in most industries.

Modbus is a request/reply protocol and offers services specified by function codes. These function codes are elements of the Modbus request/reply protocol data units (PDUs).

There are three categories of Modbus:

• Modbus Serial – either Modbus ASCII or Modbus RTU
  • Modbus ASCII – each eight-bit byte in a message is sent as two ASCII characters, main advantage is that it allows time intervals of up to one second to occur between characters without causing an error.
  • Modbus RTU – each eight-bit byte in a message contains two four-bit hexadecimal characters, main advantage is its greater character density allows better data throughput than ASCII for same baud rate.
  • Other Serial Modbus – several manufacturers have made modifications to the Modbus protocol to meet their specific application needs, these would include a Modbus Daniels, Modbus Omniflow, Modbus Tek Air and others.
• Modbus Plus – a peer-to-peer industrial network in which any controller can initiate transactions with the other controllers
• Modbus TCP – Modbus over TCP/IP Ethernet network.

Available Modbus Drivers from FieldServer Technologies

· Modbus RTU, FS-8700-01 (Standard on all FieldServer Gateways)
· Modbus ASCII, FS-8700-08
· Modbus Daniels, FS-8700-15
· Modbus Omniflow, FS-8700-33
· Modbus Tek-Air, FS-8700-32
· Modbus Plus, FS-8700-02
· Modbus TCP, FS-8704-03

Modbus Data Model

Controllers communicate using a master-slave technique, in which only one device (the master) can initiate transactions (queries). The other devices (the slaves) respond by supplying the requested data to the master, or by taking the action requested in the query. Typical master devices include host processors and programming panels. Typical slaves include programmable controllers.

The master can address individual slaves, or can initiate a broadcast message to all slaves. Slaves return a message (response) to queries that are addressed to them individually. Responses are not returned to broadcast queries from the master.

The Modbus protocol establishes the format for the master's query by placing into it the device (or broadcast) address, a function code defining the requested action, any data to be sent, and an error-checking field. The slave's response message is also constructed using Modbus protocol. It contains fields confirming the action taken, any data to be returned, and an error-checking field. If an error occurred in receipt of the message, or if the slave is unable to perform the requested action, the slave will construct an error message and send it as its response.