When PLC modules are connected over the network, the standard communication protocols are used.

The different types of standard communication protocols support different speed (baud rate), distance(network length) and the number of connecting devices (nodes).

Baud Rate is
the number of bits per second that are being transmitted or received. Common
values (speeds) are 1200, 2400, 4800, 9600, 19200, and 38400.
Start bit : This is a synchronizing bit added just before each
character we are sending. This is considered a SPACE or negative voltage or a 0.
Stop bit :. This bit tells us that the last character was just
sent. This is considered a MARK or positive voltage or a 1.
Parity bit is added to check whether corruption has occurred.
Common forms of parity are: None, Even, and Odd. During transmission, the
sender calculates the parity bit and sends it. The receiver calculates parity
for the character and compares the result to the parity bit received. If the
calculated and real parity bits don’t match, an error occurred and we act
appropriately.

What is the Baud Rate?

Baud rate is also known as Communication speed.

The rate of data transmission on the network is called Baud Rate. The unit of Baud Rate is bits/second (b/s). In simple terms, it is calculated as the

“number of data transferred per second”.

As all the communication happens over the
channel in 0 and 1, Baud Rate and Bit Rate are the same.

Binary Data Type Conversion for Baud Rate

Binary system based on the bit. Bit is either 0 or 1.

• 1 Nibble= 4 Bits
• 1 Byte= 8 Bits
• 1 Word= 2 Bytes = 16 Bits
• 1 Double Word= 4 Bytes= 32 Bits

Let’s see, binary data representation diagram.

This is all about topmost PLC communication protocols. And I also covered the basic fundamental parts of protocol like Baud rate (communication speed), network length and the number of nodes supported by each protocol.

 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 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 its OSI layers and have their individual functions which differentiate themselves from each other. They have been manufactured by different vendors.

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).

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.

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.

DF-1

It is an asynchronous byte-oriented protocol that is used to communicate with only Rockwell devices based on RS-232.

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.

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.

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.

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.

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.

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.

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.

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.

RS-232 Communications

RS-232 is an asynchronous communications method. We use a binary system to transmit our data in the ASCII format. PLCs serial port is used for transmission/reception of the data, it works by sending/receiving a voltage, With RS232, normally, a 1 bit is represented by a voltage -12 V, and a 0 by a voltage +12 V. (The voltage between +/- 3 volts is considered There are 2 types of RS-232 devices.)
DTE – Data Terminal Equipment and a common example is a computer.
DCE – Data Communications Equipment and a common example is a modem.
PLC may be either a DTE or DCE device. When plc and external device are both DTE, (or both DCE) devices they can’t talk to each other. The solution is to use a null-modem connection. Usually, the plc is DTE and the external device is DCE. Using RS-232 with PLC Some manufacturers include RS-232 communication capability in build with the main processor and Some uses the “programming port” or External additional pluggable RS232 expansion Module.
External device may be an operator interface, an external computer, a motor controller, a robot, a vision system, etc. To communicate via RS-232 we have to setup:
1. Where, in data memory, will we store the data to be sent?
2. Where, in data memory, will we put the data we receive from the external device?

RS-485 Communication

RS-485 is one of multi-drop and two wire type communication that allows us to communicate to multiple devices at the same time. According to the standard, up to 32 devices can be connected at the same time. Maximum distance from end to end can be up to 1200 meters. Many PLC allow
connecting up to 128 slave nodes. By using repeaters, however, both the total number of devices and maximum distance can be extended. RS-485 network can be used as a two-wire or four-wire network. The four wire network would be bidirectional (a simultaneous twoway conversation can happen) whereas the two wire network works only in one direction. The RS-485 disadvantage is that it is harder to program, because it uses the same 2 wires to send and receive data. And in any given network, only one node can transmit data, other nodes can only receive at that particular moment. On the advantages side, it supports long distance communications with no problems. It also uses lower interface signal levels than the RS-232, which makes the interface circuit harder to damage.