Modbus Communication is a Communication Protocol used in PLCs for Communication between a Master/Slave. Modbus is used for transfer of information over the Serial lines between the Master and Slave. Modbus is just a set of rules for data transmission. Modbus protocol originally published by Modicon (now Schneider Electric) in 1979 for its programmable logic controllers (PLCs).
Modbus supports communication among many devices connected to the same network. Modbus was previously mainly used on wired serial communication lines, there are also extensions to the standard for wireless communications and TCP/IP networks.
Modbus Implementation Concept
MODBUS has a unique addressing pattern. A Modbus device will store every value in it at a particular address.They are listed as,
Device address
Modbus address
Description
1…10000
address – 1
Coils (outputs)
10001…20000
address – 10001
Inputs
40001…50000
address – 40001
Holding registers
In Modbus RTU, data is coded in binary, and requires only one communication byte per data byte. This is ideal for use over RS232 or multi-drop RS485 networks, at speeds from 1,200 to 115Kbaud. The most common speeds are 9,600 and 19,200 baud. Modbus RTU is the most widely used industrial protocol, so most of this paper will focus on Modbus RTU basics and application considerations.
Modbus/TCP is simply Modbus over Ethernet. Instead of using device addresses to communicate with slave devices, IP addresses are used. With Modbus TCP, the Modbus data is simply encapsulated inside a TCP/IP packet. Hence, any Ethernet network that supports TCP/IP should immediately support MODBUS/TCP. More details regarding this version of Modbus will be covered in a later section entitled “MODBUS Over Ethernet.”
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