PLC Communication protocols

let’s start plc communication. There are lots of information inside a PLC. Information such as logical arithmetic calculations or the input and output status of a device is stored in PLC’s data memory.
Data memory are internal memory registers of a PLC and each of its own memory address.
These data are accessible from external systems via communication ports built in a PLC.
Usually, a PLC will have a 9-Pin serial RS232 port with Modbus included as one of the plc communications protocols.
Optionally, there may be Ethernet ports or various filed buses for protocols such as DeviceNet, Canbus, Profibus, Modnet, Profinet.
For example, the run state of a motor is available to the PLC via input Number 4 Depending on the make of the PLC, the register address for input 4 may be mapped to the Modbus address 10004.

Basic understanding plc communication we required to understand terms



  • ASCII: ASCII is the American Standard Code for information interchange let’s look at what that means and how you can use it as a key is based on a simple idea of using
  • numbers to represent text the block of text can be represented entirely by numbers and thus easily stored in a computer’s memory. the exclamation mark is always assigned 33 and 33 always references exclamation mark there are other characters defined by ASCII
  • ASCII IS 7-bit code.ASCII is Used to represent English Language character(latter, number, symbols, and punctuations) by the binary number used in computer
  • ASCII is also commonly used in computer science particularly in programming text files and data conversions programmers might use ASCII different devices can communicate with each other as long as both use ASCII code

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Distributed Control :

  • Distributed Control: This is an automation concept in which portions of an automated system are controlled by separate controllers.A process manufacturing plant operates with hundreds or thousands of field instruments, valves and operator
    interfaces that all need to work together efficiently and reliably. At the heart of it all is the Distributed

We already discuss SERIAL COMMUNICATION my previous article that is one of the parts of plc communication

  • Baud: unit of signaling speed so.Baud rate: number of symbols transmitted per second (s) Expression: Baud rate = bit rate / the number of bits per signal unit
  • Bit: Unit of information so.Bitrate: number of bits transmitted per second (b) Expression: Bit rate = baud rate x the number of bits per signal unit
  • Kbps. This stands for “thousand bits per second,” which is a rate of measure for electronic data transfer.
  • Mbps. This stands for “million bits per second.
  • Parity: bit may be transmitted to check errors
    even parity: Number of 1 bits is odd
    even parity: Number of 1 bits is even


  • Start/Stop bits: We send 1 or more stop bits at end of each cycle or 1 start bit at the beginning There may be a gap between each cycle extra start-stop bits and gaps are to alert the receiver and allow it to do synchronize with a data stream

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3.CIP Protocol :

  • CIP Protocol: The Control and Information Protocol (CIP) provides connections between higher-level devices (controllers) and industrial devices (sensors, actuators).FOUR MAJOR NETWORKS,

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