MODBUS, RS 232, RS 485, RS 422 and Cable screen

 Mod Bus : It is transmission protocol for process control system in context to other buses. There is no transmission medium is defined.

It defines a digital communication network to have one master and one or more slave devices. Either a single or multi-drop network is possible. All message transactions are initiated by MASTER. Euro-therm instruments communicate using the MODBUS RTU binary protocol.

The JBUS protocol is identical in all respect but one to the modbus protocol. The one difference concerns the parameter or register address. Both use numeric index but JBUS index starts at ‘0’ while MODBUS index starts at ‘1’.

MODBUS TCP allows a host computer to connect to a remote serial port and to talk to  a MODBUS communicating device via an interconnecting Ethernet LAN using special MODBUS TCP driver. AS far as the slave instrument is concerned, The transaction is standard MODBUS transaction. It has 1-247 slaves.

Note: If you want sample master slave message transaction or information it carry then send me mail or comment your queries below.

The protocols that user can choose are

1.       RS  232

2.       RS 485

3.       RS 422

4.       Cable screen

RS 232

This standard alows a single instrument to be connected to a pc, a Programmable Logic Controller or similar devices using a cable length of less than 15 m.

RS 422

Strictly speaking, RS 422 is a standard permitting  ‘point to point’ connection between two communicating instruments using a full duplex, differential signals on two pairs of wires.

In principle, therefore, an RS 422 link only aloows a single instrument to be connected to a PC. However, Many instrument provide an enhanced version of RS422 that also meet full electrical requirements of RS 485 described above. This allows upto 31 units of loads to be connected on the same network, But only with 5 wire electrical connections. The transmission and reception of data use two pairs of twisted cable, With a separate cable provided as circuit common.

Pins of RS 232 and RS 422 hardware ports

RS 485

This standard allows one or more instruments to be connected (Multi-dropped) using a two wire connection, with cable length of up to 1200 m. A maximum of 31 unit loads or instruments and one master may be connected in this way. The balanced differential signal transmission is less prone to interference and should be used in preference to RS 232 in noisy environments. RS 485 is recommended for plant installation. Although RS 485 is commonly referred to as a two wire connection. A separate cable is often provided for linking the circuit commons.

Difference between RS 232, RS 422 and RS 485 is shown below:

Transceiver model of RS 232, RS 422 and RS 485 is shown below. You should use as your requirements in Automation industry.

Cable screen

The use of screened cable with the screen/Shield connected to earth is recommended as it provides additional protection against noise. To prevent earth-loop problem, The cable screen should be connected to electrical earth at one point only- normally at the supervisory computer end of the cable. The screen must not be used as circuit common.

We will discuss the LAN configuration tomorrow which is very much important as field buses.