Modbus

What is Modbus?

                 It is a serial communication protocol which is developed by modicon company in 1979 for their PLC's. Modbus can be operate over Serial as well as TCP/IP. At that time it was used for only modicon PLC's but now a days it used as an open protocol which is used by wide range of automation products.

       For Serial it uses a terminology like                                 For Ethernet it uses a terminology like 

                Master ------------- Slave                                                   Client ------------- Server 

                 There are three major variations in modbus protocol

                        1. Modbus ASCII

                        2. Modbus RTU

                        3. Modbus TCP/IP

                This Modbus protocol originally developed using ASCII codes to encodes the messages and this version of protocol still used in todays days.

                Modbus RTU is by far most common implementation uses. There are mostly two types of methods used for checking.
                        1. Binary coding
                        2. CRC error checking

                 Modbus ASCII and Modbus RTU are two different variations in modbus protocol, so the device configured for ASCII mode cannot communicate on using RTU. It means the device which is configured for ASCII mode cannot used in RTU mode.

                 RTU uses one of three typical electrical interface,                                  

                     1. RS232 is a simple point to point communication. If you need to connect only one slave device at that time you can use this communication. It wont allow more than 50ft or 15m.

RS 232 communication

               2. If the device more than 50ft or have to connect devices more than 1 you should use RS485 or RS422. Possible speeds are 300 to 100000 bits/sec. Normally speed varied between 9600 - 19200 bits/sec, it is termed as baud rates.

                 A Modbus serial network has a master device that issues command to slave devices, the slave device will not transmit any information unless they receive a command to do so from master. There can be only 1 master in a n/w and maximum of 247 devices(slave). Each with a unique slave ID from 1 - 247.

                 RS485 can not drive more than 32 nodes in a single segment . If in a particular application we required the nodes more than 32 then repeaters will required.

One point should be noted here that connection should make in daisy chain manner only.   

• Modbus TCP/IP network

Modbus over ethernet works exactly like it sounds. It uses regular ethernet cables and switches to communicate with each other. It uses client and server terminology as we see in beginning.

The big difference between Modbus TCP/IP is that, a MBAP header or modbus application header is added to the start of each message i.e.,

As seen in above picture normally, Modbus message looks like above format but for TCP/IP it is modified as below,

                 This total topics discussing on 

                        1. Modbus RTU

                        2. Modbus TCP/IP using MBAP

                 Modbus TCP/IP uses the terms clients and server instead of master and server. 

                 TCP/IP network consist of client connected to a switch or series of switches to which all the servers on the network are also connected.

                 Modbus TCP/IP uses internet protocol addressing in which it uses IP address and subnet mask which is shown below...

                 Internet Protocol addressing

                 IP address 000.000.000.000 -------  This is the location of particular devices on the network

                 Subnet mask 255.255.255.0 -------  This used for served to simplify the task of routing traffic within the network.

                 Now lets see the Modbus important addressing system and the different data-tables.

Coils                          - Read/write       00001-09999

Discrete inputs          - Read only        10001-19999

Input registers           - Read only        30001-39999

Holding registers      - Read/write       40001-49999

                 There are four tables where the information is stored. Two tables stored the discrete value called as coils and two tables stored the numeric. 16 bit value known as registers.

                 For each type of data, there is 1 read only table and 1 read/write table. There is no table for 32 bit data type.

                 When Modbus designed double integer and float value are not available in PLC's there is a way to use these data types are, Each table has 9999 addresses.

Data type table from 00001 - 09999 are the read/write coils.

Data type table from 10001 - 19999 are the read only discrete inputs.

Data type table from 20001 - 29999 are the read only input registers.

Data type table from 30001 - 39999 are the read/write holding registers.

Coils and Discrete i/p's               =      1 bit        =    In Rockwell terms BOOL (This mean ON or OFF)

Register is the term for 1 word   =      16 bit      =    In Rockwell terms INT

                 No word for floating point and double integer but these can be send by dividing them into two registers.

                 Float value are any real number with a decimal point that is represented by 32 bit registers.

Ex. 14.44434 = 32 Bit of data

Double integer     {16 bit + 16 bit}     32 bit

Solution:- 

In slave                       In master                   In Rockwell processor

32 bits                         16 bits                    32 bits Real or DINT tag

                                    16 bits

Following are some function codes used in Modbus:- 

1. Read coil status

2. Read input status

3. Read holding registers

4. Read input registers

5. Write single coil status

6. Write single register

15. Multiple coil write

16. Multiple register write

Last summary,

The master gives command to slave with following functions

Demo on Modbus

1. Click here

2. Click here