PLC Communication Services

A PLC system is configured using several networked components, such as PLCs, computer, communication modules, and HMI, with communication interfacing capabilities. These components exchange data between two or more communication partners through the network using specific protocols for handling the data traffic.

A network is characterized by the type of hardware involved, communication procedure, and the data transfer rate. Accordingly, PLC communication networks can be classified as multi-point interface (MPI), Profibus, Industrial Ethernet, and point-to-point (PTP) connection.

Multi-Point Interface (MPI)

Multi-point interface (MPI), used in SIEMENS PLC systems, is designed as a programming device interface and is intended for networking a small number of CPUs to exchange small volumes of data.

PROFIBUS

Profibus complies with the standard EN 50170 and is used for networking field devices for communication. This type of connection is a local area network that provides a high-speed data communication bus to which devices within the local area can be connected.

Industrial Ethernet

Industrial Ethernet is a network for connecting PLCs and computers. It is suitable for the high-speed exchange of large quantities of data and facilitates communication between the communication partners.

Point-to-point (PTP) Connection

In a point-to-point connection, a communications processor forms the interface between a PLC and a communications partner. Serial communication occurs when bits belonging to each byte of information are transmitted one after the other in a serial order.


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Cables

Cables are used as the transmission medium for communication among devices connected in a PLC system. There are two types of cables available. They are: (1) Copper cables and (2) Fibre-optic cables.

  • Copper cable can be sub-divided into: (1) 2-core cable, (2) coaxial cable, and (3) shielded twisted-pair cable
  • Fibre-optic cable can be sub-divided into: (1) glass fibre-optic cables and (2) plastic fibre-optic cables

RS-232C

RS-232C interface is a kind of serial physical interface standard. This method is based on 1:1 communication, which is generally applied to the point-to-point communication within a short distance of 15 meters.

RS-485

RS-485 is a network interface that uses a master-slave communication method. That is one master communicating with multiple slaves (1: N). This network can be used as a two-wire or four-wire network. RS-485 network supports long-distance links, and the cable length can be up to 1200 m.

RS-422A

RS-422A is a network interface that also uses a master-slave communication method. This network can be used as a two-wire or four-wire network. RS-422A network supports long-distance links, and the cable length can be up to 1200 m.

Authored by

Joji Parambath

Author

Fluid Power Educational Series

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