Introduction to Modbus I

Modbus is a communication protocol used for transmitting information between electronic devices over serial lines. It is commonly used in process and factory automation.

MODBUS was developed by Modicon (now Schneider Electric) in 1979 as a means for communicating with many devices over a single twisted pair wire. It was created specifically for use in Modicon PLC's for industrial applications. 

Today it is an open protocol, used by a wide range of automation products. Modbus can be used over Ethernet as well as serial cable. 

There are 3 major types of variations of Modbus Protocols.

• Modbus ASCII 
•  Modbus RTU.
•  Modbus TCP/IP. 

Modbus was originally developed using ASCII characters to encode messages and this version of the protocol is still in use today. Modbus RTU, the most common implementation using binary coding and CRC error checking.

The modes are incompatible so a device configured for the ASCII mode cannot communicate with one using RTU. 

Modbus RTU devices typically use one of the three electrical interfaces-

• RS232 
• RS485
•  RS422.

RS232 is a simple point to point arrangement. In this, we one connect one device to another device and the distance between the two devices is less than 50 ft or 15m, then RS232will do this job.

The original scheme ran over RS232 but was adapted to run on RS485 to gain faster speed, longer distances, and a true multi-drop network.                 

When the distance is more than 50ft, we use RS485 & RS 422