In the real world of device to device process automation communication,
a dialogue or conversation between devices occurs systematically,
in many cases, over different types of communication networks and in different languages.
Depending on the process automation equipment manufacturer,
a very specific or proprietary language is used
or may be a language that is commonly open to the industry.
It is these open protocols many manufacturers adapt
to easily integrate their products in a market.
An “Open protocol” means the specifications are published
and may be used by anyone freely or by license.
Open protocols are usually backed by a combination of corporations,
user groups, professional societies, and governments.
This provides users with a much wider choice of devices or systems
that can be utilized to meet specific applications.
Advantages of open protocols include support by multiple manufacturers,
software vendors, and install/service organizations,
active community groups for support,
the ability to stay current and add capabilities in the future.
One of the most common automation communication protocols
of connecting industrial electronic devices used today is Modbus.
In this lesson, we will discuss what Modbus communication protocol is
and the types of Modbus communication used in the industry.
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The Modbus communication protocol is the oldest
and by far the most popular automation protocol
in the field of process automation and SCADA
(Supervisory Control and Data Acquisition).
Knowing how to create Modbus based networks
is essential for any electrical technician
and engineer working in these occupation fields.
Being able to integrate devices from different manufacturers
is a skill that is in demand
and will ultimately make you more valuable and marketable in the industry.
Modbus is a communications protocol
published by Modicon in 1979
for use with its programmable logic controllers (PLCs).
Modicon is now owned by Schneider Electric.
Modbus provides common language
for devices and equipment to communicate with one another.
For example, Modbus enables devices on a system
that measures temperature and humidity connected on the same network
to communicate the results to a supervisory computer or PLC.
And the development and update of Modbus protocols
have been managed by the Modbus Organization.
The Modbus Organization is an association of users and suppliers
of Modbus-compliant devices.
Several versions of the Modbus protocol exist for the serial port
and Ethernet and the most common are Modbus RTU,
Modbus ASCII, Modbus TCP and Modbus Plus.
Modicon published the Modbus communication interface
for a multidrop network based on a master/slave architecture.
Communication between the Modbus nodes is achieved
with send request and read response type messages.
Modbus is an open standard that describes
the messaging communication dialog.
Modbus communicates over several types of physical media
such as serial RS-232, RS-485, RS-422 and over Ethernet.
The physical media will be selected at the time when you purchase the devices.
The original Modbus interface ran on RS-232 serial communication,
but most of the later Modbus implementations use RS-485
because it allowed longer distances,
higher speeds and the possibility of multiple devices on a single multi-drop network.
Master/Slave Modbus communication over serial RS-485 physical media
showing two-wire transmit and receive connections.
On simple interfaces like RS-485 and RS-232,
the Modbus messages are sent in plain form over the network
and the network will be dedicated to only Modbus communication.
However, if your network requires multiple heterogeneous devices
using a more versatile network system like TCP/IP over ethernet,
the Modbus messages are embedded in Ethernet packets
with the format prescribed for this physical interface.
So in this case, Modbus and other types of mixed protocols
can co-exist at the same physical interface at the same time.
The main Modbus message structure is peer-to-peer.
Modbus is able to function on both point to point and multidrop networks.
Modbus devices communicate using a master/slave (client-server for Ethernet) technique
in which only one device can initiate transactions (called queries).
The other devices respond by supplying the requested data to the master,
or by taking the action requested in the query.
A slave is any peripheral device such as an I/O transducer, valve, network drive,
or other measuring types of devices which processes information
and sends its response message to the master using Modbus.
Masters can address individual slaves
or initiate a broadcast message to all slaves.
Slaves return a response to all message queries addressed to them individually,
but do not respond to broadcast messages.
Slaves do not initiate messages on their own
and only respond to message queries transmitted from the master.
The master’s query will consist of a slave address (broadcast address),
a function code with a read or write data command to the slave,
along with the write command data
if a write command was initiated by the master,
and an error checking field.
The error checking is a value the master or slave creates
at the beginning of the transmission or response
and then checked when the message is received
to verify the contents are correct.
A slave’s response consists of fields confirming it received the request,
the data to be returned, and an error checking data.
If no error occurs, the slave’s response contains the data as requested.
If an error occurs in the message query received by the slave,
or if the slave is unable to perform the action requested,
the slave will return an exception message as its response.
The error check field of the slave’s message frame
allows the master to confirm that the contents of the message are valid.
In this video you learned about what Modbus communication protocol is
and the types of Modbus communication used in the industry.
In the next video, we will discuss the characteristics
of how Modbus communication protocol works between devices.
Stay tuned!
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