The most widely used protocol in all of industrial automation. A very simple-to-understand and easy to implement device representation and standard serial communications make Modbus very popular. On a Modbus network every device looks to the network as a collection of coils (bits) and registers. A Modbus Master reads and writes these coils and registers using a very simple, concise command set. Communication flows in only one direction at a time. A Modbus Master sends a request. A Slave answers the request.
NetBurner Modbus TCP Product Summary:
All NetBurner products that have a serial port and support the standard NetBurner development software support Modbus RTU including all Core Modules and Serial-Ethernet Converters. Contact Real Time Automation for Modbus solutions.
Modbus is a Master-Slave communication network in which every Modbus Slave device looks to the Master as a set of coils (bits) and Registers. Modbus Coils are the Slave’s discrete inputs and outputs while Registers are 16-bit, unsigned values. Registers can be connected to real world analog data points. There are no requirements for the number and type of coils and registers supported by a Modbus device.
Modbus can be implemented on any device with a serial port making it available to any 8-bit microprocessor. Though it can be used in a Single Master/Single Slave configuration over RS232 communications it is usually implemented using RS485. RS485 is a multi-drop, daisy-chained communications standard, normally used with 32 nodes or less. In an RS485 network every Modbus Slave device is identified with an 8-bit address. Address 255 is reserved and used for broadcast communications.
Modbus uses a simple Master/Slave communication mechanism. A Modbus Master issues a message to a specific slave. The message requests the value of some number of coils or registers or sets some coils or registers of a particular slave on the network. The command is received by every Slave on the network and replied to by the slave with the address matching the message address. A Modbus Master also has the capability to issue “broadcast” write messages to all slaves on the network using address 255.
The Modbus message set in practice is extremely concise. Though there are twenty-some commands only a few are actually ever implemented. The most common commands are read coils, write coils, read registers, set register and set multiple registers. The small command set is another reason for it’s great popularity in the world of small control devices