How does Modbus Communication Protocol Work?


In the previous video, you learned about
what Modbus communication protocol is and the types of Modbus
communication used in the industry. Now, as promised, in this video
you are going to learn about How Modbus communication
protocol works between devices. Let’s get into it. before we get into today’s video if you love our videos, be sure to click the
like button below. and, make sure to click subscribe and the little bell to receive
notifications of new RealPars videos. This way you never
miss another one! Each Modbus message has
the same structure. Four basic elements are
present in each message. The sequence and order of these
elements are the same for all messages. This allows for easy parsing of the
content of the Modbus message. A conversation is always started
by a master in the Modbus network. A Modbus master sends a message, and depending on the
contents of the message, the slave interprets the
message and responds to it. Physical slave addressing
in the message header is used to define which slave
device should respond to a message. All other nodes on the Modbus network ignore the message if the address field
doesn’t match their own address. Modbus functions
perform read and write instructions to the slave’s
internal memory registers to configure, monitor, and control
the slave’s inputs and outputs. Modbus devices will typically
include a register map outlining where the configuration, input and output data can
be written and read from. You should always refer to the
slave’s register map of your device to gain a better understanding
of its overall operation. The Modbus data model has a simple structure
described in four basic data types: Discrete Inputs, Coils Outputs,
Input Registers (Input Data), and Holding Registers (Output Data). The service request area of the message or Modbus Protocol Data Unit or PDU
is comprised of a function code, and a number of data bytes
requested by the master. The Modbus memory
registers of a device are organized around the four
basic data reference types and this data type is further identified by the leading number used in
the devices memory address, such as, a 0 based register
referencing a message to Read or Write discrete
outputs or coils, or a 1 based register referencing
Reading discrete inputs, or a 3 based register referencing
Reading input registers, and a 4 based register referencing Reading
or Writing to output or holding registers. The function code field specifies
which register data group it reads or writes to
and from the slave. For example, a Function code 03,
read Holding Registers 40001-40002 is addressed as data register 0000 in the data address field of
the message sent to the slave. The function code 03 works on this Holding
Register type in the slave’s data map because the request specifies using a
holding register data type operation and this type of addressing
in the request is implied. The fields in the PDU are broken down
into bytes and grouped by the field name. The request message contains
Function code of 03, the starting address HI and
LO bytes of address 0000 and the count number of addresses
to read from the slave, register HI and LO bytes of count value
0002 specifies the starting register and quantity of registers
to be read from the slave. Example of a request to read the first two
registers in the Holding Register area 0…1 (register 40001 to 40002) and therefore, as another example, the holding register 40108
is actually addressed as register 108 in the message
data area of the PDU. Many of the data types are named
from its use in driving relays, for example, a single-bit
physical output is called a coil, and a single-bit physical input is
called a discrete input or a contact. The function code field of the message will contain one byte that tells the
slave what kind of action to take. Valid function codes are
range from 1 to 255, but not all codes will apply
to a particular slave. This table highlights a subset
of standard Modbus functions. In addition, the master
request data field, provides the slave with any additional
information required by the slave to complete the action specified by the
function code in the master’s request, typically including the
slave map register address, the number of registers to
provide in the request, and any write data from the master. The slave’s normal response simply echoes
the original function code of the request, but the slave’s error
response returns a code that is equivalent to the
original function code with the most significant
bit set to logic 1. For example, the Read
Holding Registers command has the function code with 1 byte
containing 8 bits is binary 0000 0011. If the slave device accepts
the request without error it will return the same
code in its response. However, if an error occurs, the slave will
return 1 byte containing 8 binary bits 1000 0011 in the function code field and appends a unique code in the
data field of the response message that tells the master device
what kind of error occurred, or the reason for the error. This function code 01, Read Coils code, is used to read from 1 to
2000 contiguous registers for the status of coils in a slave device. The Request PDU specifies the starting address
of the slave’s memory address of the first coil, and the number of coils to
read from the slave device. The function code 02, Read
Discrete Inputs code, is used to read from 1 to 2000 contiguous
status of discrete inputs in a remote slave. The Request PDU specifies
the starting address of the slave’s memory
address of the first input, and the number of coils to
read from the slave device. The function code 03, Read
Holding Registers code, is used to read the contents of a contiguous
block of holding registers in a remote slave. The Request PDU specifies the
starting register address and the number of registers to
read from the slave device. The function code 04, Read
Input Registers code, is used to read from 1 to 125 contiguous
input registers in a remote device. The Request PDU specifies the starting
register address and the number of registers. The function code 05,
Write Single Coil code, is used to write a single output to either
ON or OFF in a remote slave device. The function code 06, Write
Single Register code, is used to write a single holding
register in a remote slave device. The Request PDU specifies the address of the
slave memory register address to be written to. The function code 15, Write
Multiple Coils code, is used to force each coil
in a sequence of coils to either ON or OFF in
a remote slave device. The Request PDU specifies the coils
memory address to be forced ON or OFF. And the function code 16, Write Multiple registers is used to
write a block of contiguous registers (1 to 123 registers) in
a remote slave device. While these Modbus function codes represent
the most common read and write functions, it would be helpful for you to review the Modbus
specification for additional information. For additional detailed Modbus
protocol technical information please refer to the
modbus.org website. Communication with slave devices
or master PLCs or computers can be accomplished with
Modbus simulator software to run on your personal computer. The connection can be
either serial or ethernet and in the form of a master or slave. The software will allow you to
perform all of the Modbus Protocol Communication function code to simply
read or write to an existing slave. You can set up one PC to run
the slave simulation software and another to run the
Master simulation software. Connecting to a slave is first performed by setting the communication
parameters for your serial COM port. And then by entering the slave’s
address in the Device ID field, for example 1, selecting
the function code 01 to read the slave’s Coil Status stating at address 0001 and
reading a length of 100 coils. In addition, there are several companies
providing products and support to help you with
protocol communication. ProSoft Technologies, HMS
Industrial Communication, and MOXA are to name just a few. This concludes the video,
Modbus Communication protocol. I hope you have learned what’s
required to move forward in creating your own
motion control project. If you enjoyed this video,
please press the like button. This video is one of a series of
videos on motor and motion control, so please check back with us soon for more motion control topics. Want to learn PLC programming
in an easy to understand format and take your career
to the next level? Head on over to realpars.com

35 Replies to “How does Modbus Communication Protocol Work?

  1. Very good video. Can you make a video that how to communicate weintek hmi with mitsubishi vfd via rs486 communication without plc. Just to control the speed directly from weintek.hmi

  2. Sir your team is really doing a wonderful job… The video is so informative..
    Sir request you to please explain us how to evaluate P,I and D values in stand alone pid controller.

  3. IS it possible to use real time clock in siemens s7-300 so that I can take output from clock and turn off unwanted motors running
    [for example
    From 11.00AM to 11.30 AM my machine should turn off]

  4. Hello RealPars..!
    All your videos are extremely useful and content full. Sir Pls make some videos on OPC Server or OPC/DA Server and why and where it is used.??? How to configure it.

  5. Hello Real Pars!!..
    I am very much thankful for all your videos.
    Kindly upload some more Industrial Communication Systems. Also PL make some videos on OPC AE and OPC DA System in details with example.
    Also how we can communicate external world with DCS/SCADA System.
    once again thanks…!!

  6. Thank you a lot guys for these videos. I'm writing my thesis about Modbus protocol and your videos are helping me so much to deal with this problem. Many thanks again! 😉

  7. Hi, thanks for this video
    I searched for the modbus technical information but I couldnt find it, can you provide the link for it

  8. All video are so good and helpful Sir..please make some videos of modbus plc program for field instruments in s7300 plc.

  9. Thank you very much. These presentations and symbols are very useful and helps understanding the concepts much easier. What is the software you use to create these presentations ? Like the HMI symbols and PLCs etc 👏👏

  10. So if i am correct you can read any register via modbus? Isnt it a bit dangerous? You can read in that case vulnerable parts of chip memory. Or is it just a somekind of fasade what will be mapped inside memory of real device?

  11. Great job n informative as always Real Pars!
    Hope you continue videos series like this for all Industrial comm. protocols like ASI protocol, ProfiNet, Profibus ….etc

  12. Hi, really appreciate for your time, knowledge and effort in the process of educating us by providing simply superb demonstrations. Thank you so much .
    at 9min 18 sec video, for "most common function code" for 02 instead of read inputs captured as read outputs, kindly correct me if im wrong

  13. Do you have website to learn more about modbus rtu i already watch this video but i need more can you give another thank you any website so that i can understand more

  14. Real pars u r more than excellent. U r awesome. I love ur videos. Plz plz upload videos on CAN open i/o' s and its communication. Also upload more explanatory videos of DCS, it's communication protocols and some of its troubleshooting tips. Thanks alot for ur time and wonderful videos. U r love of every Automation technician

  15. Real Pars,
    We do really appreciate your efforts and time to provide these learning videos , simple, informative in a very short of time
    Well done
    Hamza

  16. Dear Sir, Thank you so much for the crystal clear tutorial. We all love to learn from your videos. I have one question, Sir. How can I get a copy of the simulation software for the slave devices? Would you mind kindly reply me the link if you have free time, Sir.

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