21EI601 INDUSTRIAL AUTOMATION
UNIT 5 LP 3
COMMUNICATION STANDARDS IN PLC - ETHERNET
Communication is an important role player in PLC. Apart from traditional
hardware IO’s, communication protocols provide more diversity and
flexibility to exchange data with various means.
Figure 1. Communication Protocol
1. Ethernet/IP
It is a protocol that works under Common Industrial Protocol (CIP), which is
an open application layer protocol. It is an advanced version of
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standard Ethernet, which is useful only for in-home and commercial purposes;
but not for industrial applications. It defines all the devices on a network as a
series of objects and binds all of them to work on the same standard. This
protocol was developed by Rockwell Automation.Similar protocols with these
standards are Profinet, ControlNet, DeviceNet and RAPIEnet. They are
different in it’s OSI layers and have their individual functions which
differentiate themselves from each other. They have been manufactured by
different vendors.
2. Modbus
Modbus is a protocol that is used for transmitting information over a serial
lines or Ethernet, based on master-slave technology. Here we have two types
of devices in this communication. The devices which request information are
called Modbus Master and the devices which provide the information are
called Modbus Slaves. It is further categorized into Modbus RTU, Modbus
ASCII, and Modbus TCPIP. This protocol was developed by Modicon (now
Schneider Electric).
3. Profibus
It is similar to Modbus RTU, which also works on serial line communication.
The only difference is that it is owned by Siemens Automation.
4. Optomux
It is a serial network protocol that works on RS-232/RS-485 and works on
ASCII standard. It was developed by Opto22. Similar protocols with these
standards are Interbus and DirectNet.
5. 6DF-1
It is an asynchronous byte-oriented protocol that is used to communicate with
only Rockwell devices based on RS-232.
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6. Interbus
It is a serial network protocol which works on RS-232/RS-485 and works on
RTU standard. It was developed by Phoenix Contact.
7. HostLink
It is a serial network protocol that works on RS-232/RS-485 and works on
RTU standard. It works only on Omron PLCs. It was developed by Omron.
8. Data Highway (DH+)
It is a protocol developed by Rockwell Automation and uses transformer-
coupled differential signals; meaning that the transmitter and receiver stations
need not be at the same ground potential. It works with the differential
signaling concept. It uses two wires for data transfer and the data is
represented with voltage differences in the two wires. Here the data is carried
with differential voltages, the noise will be easily removed in the two wires. It
uses half-duplex transmission for communication. It is a very old protocol and
is used only in Rockwell PLCs. Nowadays, it has become obsolete and rarely
used. It works on token-passing protocol and uses trunk lines with drops.
9. Point to Point (PP)
As the name suggests, it is a communication protocol which is used to
communicate between only two connected devices.It is byte-oriented and is
full duplex.
10. Actual Sensor Interface (ASI)
It is a protocol that is used to connect all the sensors and actuators in a field
with a single two-conductor cable. It reduces the wiring and manpower done
to connect the field equipment to the PLC. It works in master-slave
technology.
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11. Open Smart Grid Protocol
This protocol has been developed to connect all the electrical devices in a
power grid through communication and make it a smart grid system.The
devices are meters, direct load control modules, solar panels, gateways, and
other smart grid devices. Due to use of communication, all the information is
transmitted in a safe and efficient way; to make the whole power grid system
a better way to operate.
12. CAN (Controller Area Network) Open
This protocol is an application layer protocol that communicates with various
devices using peer messaging. It is a multi-master
slave communication system and has an object dictionary that contains all the
functions of a device.This is an actual communication mean and has standard
communication objects for real-time data (PDO’s), configuration data
(SDO’s), timestamp, sync message, emergency message, boot-up message,
NMT message, and error control message, and other data. In a hardware
configuration, one point to remember is that it requires termination at the end
device to establish the whole link communication.
13. HART (Highway Addressable Remote Transducer)
HART is a protocol in which digital data is superimposed on the traditional
analog signal of 4-20 mA; so that the user gets both the analog information as
well as digital information. HART-enabled field transmitters and actuators are
of great use in industrial automation; as the user gets all the information and
calibration settings sitting at one corner in the office. These are the general
communication protocols used in PLC. Apart from these, many more are
available depending on different manufacturers.
