PLC Communication and Network Cables

 PLC communication

In This chapter we discuss about Siemens (PLC communication, Serial communication , Industrial network and Network Cables ) devices overview.

Chapter -16





  • Communication is central to what a PLC does.
  • Therefore understanding communication concepts is critical to understanding PLC'S.

  • PLC'S communicate with many other devices and systems in various ways.
  • In some situations a PLC connects directly to another device.
  • Often however various components of an automation system including PLC'S are interconnected using Network Technologies.

Serial Communication

  • Serial communication is the most basic form of data communication used by PLC'S because it involves transmitting data one bit at a time on a signal line and receiving data in the same fashion on another signal line or in some cases the same line.
  • A PLC uses serial communication with a variety of devices such as personal computers, printers, RFID readers, and barcode scanners.
serial communication
  • Serial communication uses interface standards such as RS-232, RS-422, and RS- 485 that identify the signal lines available for communication.
RS-422
  • All three types of interfaces can be used for a point-to-point communication.
  • RS-422 and RS-485 can also be used for a multi-drop configuration with multiple devices connected on the same set of signal lines.
RS 232

  • Connecting a PLC to another device via an RS 232, RS 422, or RS 485 interface often requires a communication module. 
RS 485
  • For example the accompanying graphic shows S7-1200 communication modules used for serial communication.
S7-1200 communication modules



Industrial Networks


  • In most factories the need for rapid information flow is critical.
  • This often requires intelligent devices such as ( PLC'S, drives, computers, HMI'S and some actuators and sensors to be interconnected by one or more local area networks.
  • A LAN is a communication system designed for private use in a limited area.
  • LAN'S are used in office areas as well as in manufacturing environments however LAN'S used in industrial applications must be able to operate reliably in conditions that are unsuitable for office equipment.
  • Issues such as data rate, the number of devices to be connected, the reliability and noise immunity required compatibility with other network    and cost are examples of important industrial network considerations.
  • Another important characteristic that defines networks is the network topology.
  • The topology of a network is the pattern formed by network devices.
  • Examples of network Topologies include

Star, Tree, Linear, Ring, and Combined Topologies.


  • Some networks allow all of these topologies and other networks are more limited in the types of topologies available.
  • No single topology is best for all applications so careful consideration should be given when choosing the specific topology for a network or portion of a network.


Network Cables

  • A variety of cable types are used for industrial network communication.
  • Two types of cables used for Industrial Ethernet including PROFINET are shielded twisted pair cable and fiber optic cable.

  • Shielded twisted pair cable consists of twisted pairs of copper wires with a layer of braided wire between the outer plastic jacket and the inner conducting wires to provide protection from external sources of electrical noise.
  • Fiber-optic cable is recommended for use in applications with high levels of electrical noise where grounding problems exist or where electrical isolation is needed.
  • These cables are also used in open-air systems or where no electromagnetic radiation is permitted.

  • PROFIBUS networks also use various types of shielded twisted pair cables.
  •  AS-Interface networks  however use a two-wire cable that caries both data and power.

Wireless Communication
  • Wireless network devices send and Receive signals through the air.
  • Wireless networks are used where devices must be mobile or where it is too expensive or difficult to run cables.
  • For example the accompanying graphic shows a mobile panel that allows an operator or maintenance person to monitor a system without being tied to a control room.


  • Wireless networks are also used where the controlled equipment must be mobile such as with automated guided vehicle systems, overhead monorails, and cranes.
  • One of the most popular types of factory wireless networks is IWLAN, which stands for Industrial Wireless Local Area Network.
  • IWLAN uses devices with antennas to transmit and receive radio wave signals.
  • There are a variety of antenna types used with IWLAN.

 
  • This includes BQ9M radiating cables, one of which is shown in the foreground in the accompanying graphic.
  • RCoax cables are used as antennas from IWLAN access points.
  • Openings in the outer conductor of the cable provide a defined Signal strength along the entire length of the cable enabling reliable radio links in areas where conventional antenna technology is impractical.
  • For example this unique antenna design allows RCoax cable to be laid in the rails of an overhead monorail track where standard antenna technology is very costly to install and maintain.


Chapter 17 we will discuss about ( Profibus DP , Actuator Sensor Communication , Industrial Ethernet , Profinet , Profinet Device , Profinet Profiles ) Topics.

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