Siemens PLC Communication and Industrial Networks | PROFIBUS DP, PROFINET, Cables & Protocols Explained

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PLC Communication & Industrial Networks | PROFIBUS DP, PROFINET, Cables & Protocols Explained

PLC communication is the backbone of modern industrial automation systems. It enables seamless data exchange between controllers, sensors, actuators, HMIs, and other intelligent devices across the network.
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Siemens PLC Communication and Industrial Networks
 
This article provides a comprehensive overview of PLC communication methods, including serial communication, industrial networks, network cabling, PROFIBUS DP, PROFINET, and Industrial Ethernet technologies. It is designed for engineers, technicians, and students who want to understand how communication systems function in real-world automation environments.

PLC Communication Overview

  • 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.
  • This diagram shows how industrial devices like PLCs, HMIs, CNC machines, and computers are connected through Industrial Ethernet PROFINET and PROFIBUS networks to communicate and control automation systems.
how industrial devices like PLCs, HMIs, CNC machines, and computers are connected
  • In some situations a PLC connects directly to another device 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 in PLC

  • 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. 
Serial Communication in PLC
  • A PLC uses serial communication with a variety of devices such as personal computers, printers, RFID readers, and barcode scanners.
  • Serial communication uses interface standards such as RS232, RS422, and RS485 that identify the signal lines available for communication.
  • All three types of interfaces can be used for a point-to-point communication.
RS232, RS422, and RS485
  • RS422 and RS485 can also be used for a multi-drop configuration with multiple devices connected on the same set of signal lines.
  • Connecting a PLC to another device via an RS 232, RS 422, or RS 485 interface often requires a communication module.
  • For example the accompanying graphic shows S7-1200 communication modules used for serial communication.
Siemens S7-1200 communication module with RS422 & RS485 interface
  • The image is illustrating the Siemens S7-1200 communication module with RS422 & RS485 interface and S7-1200 communication module with RS232 interface.

Industrial Networks in Automation

  • 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.
industrial Ethernet cable termination and connector types
  • This image  demonstrates the practical process and hardware for making reliable industrial network connections using:
  1. FastConnect stripping tools for easy cable preparation,
  2. Rugged RJ-45 connectors for standard Ethernet,
  3. M12 connectors for more demanding industrial applications,
  • These components are typically used in PROFINET, EtherNet/IP, and other industrial Ethernet networks in factories.
  • 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, and the reliability and noise immunity required compatibility with other network and cost are examples of important industrial network considerations.

Network Topologies

  • 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.
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.

Industrial 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
  • 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.
Industrial Ethernet Fiber Optic Cable

The image is illustrating the most common cable types used in industrial automation networks:

AS-i Cable
  • Yellow flat cable (AS-Interface cable)  
  • Used for simple, low-cost sensor/actuator networks in industrial automation.
Industrial Ethernet Fiber Optic Cable
  • Two black cables with fiber optic connectors (likely SC or similar).  
  • Used for high-speed, long-distance, noise-immune data transmission in harsh industrial environments.
Industrial Ethernet Shielded Twisted Pair Cable with RJ-45 Connector
  • Green cable connected to a metal RJ-45 connector (Siemens branded).  
  • Standard for Industrial Ethernet (PROFINET, EtherNet/IP, etc.). 
  • The shielding protects against electromagnetic interference (EMI) in factories.
PROFIBUS Cable with M12 Connector 
  • Purple cable connected to a robust metal M12 connector (Siemens branded).  
  • PROFIBUS is a very common fieldbus protocol used for reliable communication between PLCs and field devices.
  • 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 (IWLAN)

  • 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.
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.
Wireless Communication (IWLAN)
  • These components are part of Siemens Industrial Wireless LAN (IWLAN) portfolio, used for reliable wireless communication in factories, cranes, AGVs, and harsh industrial environments.
  • 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.

PROFIBUS DP Communication

  • A Fieldbus is an industrial network used for distributed control and the most popular Fieldbus is PROFIBUS. 
  • The two most common forms of PROFIBUS are PROFIBUS DP for Factory Automation and PROFIBUS PA for Process Automation.
A Fieldbus is an industrial network used for distributed control
  • Both types of PROFIBUS are open standards of PROFIBUS & PROFINET International Some Siemens PLC CPU models have one or more built-in PROFIBUS DP ports.
  • Siemens PLC'S also allow a communication module to be added for PROFIBUS DP communication. 
PROFIBUS is a master-slave system. 
  • A slave device communicates only when requested to do so by its master device.
  • A PROFIBUS master can control up to 125 slaves connected to the master in a linear topology. PROFIBUS uses copper or fiber optic cables but infrared communication is also supported.
  • PROFIBUS has a maximum data rate of 12 Mbps PROFIBUS has two classes of master devices. Most communication is handled by a class 1 master which is often a PLC A class 1 master controls all communication with its slaves.
PROFIBUS is a master-slave system
  • Most of this communication is handled through a cyclic polling process. 
  • A class 2 master is used for configuration, maintenance, and diagnostics and can communicate with class 1 masters and their slaves.

Actuator-Sensor Communication (AS-i & IO-Link)

  • AS-Interface ASI) is an open low-cost network endorsed by the AS International Association that simplifies the interconnection of actuators and sensors with controllers.
  • ASI replaces the complex wiring and proprietary interfaces often used for this interconnection with only two wires which transfer both data and power.
ASI network a master communicates
  • ASI is most often used where actuators and sensors are distributed throughout a machine or facility. 
  • In an ASI network a master communicates with up to 62 slaves through a cyclic polling process. The master is often a PLC with a communication module. 
  • The master slaves and 30 Volt DC power supply are interconnected by the two-wire ASI cable. 
  • I/O Link is another actuator-sensor communication system. 
  • I/O Link is an open communication standard developed by the I/O Link research group of PROFIBUS & PROFINET. 
  • International in response to the need for a simple low-cost way to allow actuator and sensor communication in a concentrated area such as for a control panel or individual machine.
  • I/O Link is a point-to-point system not a field bus. It uses a three-wire cable 24 V DC power supply and an I/O Link master. Depending on the master up to 16 devices per master are allowed.
I/O Link masters and some of the many I/O Link slave devices
  • The graphic shows two examples of I/O Link masters and some of the many I/O Link slave devices that can be used with Siemens PLC'S.

Industrial Ethernet

  • Ethernet was developed in the 1970's to network office computers. As the use of computers increased in both the office and the factory the need for common communication approaches lead to the development of Industrial Ethernet.
  • Which uses industrial switches to provide the performance and reliability needed for high-speed factory networks. An Industrial Ethernet switch is an active network component that allows multiple devices to communicate simultaneously at high speeds. 
  • Industrial Ethernet allows communication speeds up to 100 Mbps with full-duplex operation which means that network devices can simultaneously send and receive data. 
Siemens SIMATIC NET product family includes SCALANCE Industrial Ethernet switches
  • The Siemens SIMATIC NET product family includes SCALANCE Industrial Ethernet switches designed to fit varied requirements from localized to plant-wide networks. Siemens also offers an extensive array of devices for interconnecting network components via Twisted-Pair Cable, Optical Fiber, or Wireless signals.
  • For example Siemens Fast Connect system includes cables and connectors designed for demanding factory applications and assembly tools designed to speed installation.

PROFINET Communication

  • PROFINET is the open Industrial Ethernet standard of PROFIBUS & PROFINET International and the leading Industrial Ethernet standard.
  • PROFINET I/O the most widely-used form of PROFINET extends the benefits of standardized Ethernet communication to distributed field devices by simultaneously handling three (3) communication performance levels. 
Non-time-critical communication uses standard information technology protocols.
  1. Non-time-critical communication uses standard information technology protocols. 
  2. Real-time RT) communication works well for most control tasks.
  3. Isochronous real-time IRT communication handles the most time sensitive tasks such as motion control applications. 
  • Because PROFINET is an open standard it allows field devices from many suppliers to be easily connected to the network. PROFINET also protects investments in existing networks by simplifying their integration into a factory wide network. 
  • The figure shows an example of a few of the system types that can be easily integrated into the network. 
  • In a PROFINET system the number of devices is not restricted. PROFINET uses copper or fiber optic cables but also supports Industrial Wireless Local Area Network IWLAN) communications.

PROFINET Device Types

  • Devices on a PROFINET Network are connected through a switch. Siemens offers a variety of stand-alone network switches and as shown in the graphic many Siemens control devices have a built-in switch with one or more PROFINET ports.
  • Devices with two ports simplify use of a linear topology or a combined topology that includes devices connected in a linear manner.
  • The following PROFINET device types are: I/O Controller, I/O Device, I/O Supervisor, and Proxy.
I/O Controller, I/O Device, I/O Supervisor, and Proxy
  • A PROFINET system includes at least one I/O Controller with I/O Devices assigned to it. The I/O Controller is typically a PLC or other control system which configures the I/O Devices and exchanges data with them but does not control their communication. 
  • An I/O Device in this context is not a PLC input output device but a PROFINET- compatible electronic device that often also allows actuators and sensors to be connected. 
  • An I/O Supervisor which is often a programming device human machine interface or personal computer is used for commissioning, monitoring, and diagnostics.
Equipment, Engineering, Installation, and Maintenance
  • A Proxy is a device that connects a PROFINET network to a fieldbus such as PROFIBUS. Because the proxy maps the sub-network to PROFINET all the devices of the sub-network are available for PROFINET.
  • These PROFINET device types have been part of the standard since its inception. Later innovations to PROFINET IO include the following device types Shared Device and I/O Device.
  • The Shared Device capability allows two I/O Controllers to use the same I/O Device. 
  • This is accomplished easily by copying the configuration from one I/O controller to another.
  • The benefits are reduced costs for Equipment, Engineering, Installation, and Maintenance. I/O Device is short for intelligent device. The central I/O Controller has direct access to the input/output image in the local I/O Controller so communication is simple and fast and no complex programming or additional communication hardware are required.

PROFINET Profiles

  • PROFIBUS & PROFINET International PI has working groups with representatives from many companies including Siemens. 
  • These working groups specify configurations of PROFIBUS and PROFINET devices that meet special requirements. The specifications are published by Pl as profiles.
  • Profiles are open and vendor- neutral allowing users to incorporate products from multiple suppliers into their systems without concern for interoperability.
PROFINETIO, PROFIDrive, PROFISafe, PROFlEnergy,
  • In addition to the PROFINET I/O profile which provides for the integration of distributed field devices via Industrial Ethernet examples of PI profiles are: 
  1. PROFINETIO, 
  2. PROFIDrive, 
  3. PROFISafe,
  4. PROFlEnergy,
  • PROFI drive and PROFI safe were initially developed as PROFIBUS profiles and later extended to include PROFINET PROFIl energy is a PROFINET profile. 
  • The PROFI drive profile allows users and OEM'S to design machines and processes with drives from multiple suppliers communicating PROFIBUS or PROFINET PROFIsafe is an integrated safety technology applicable for both discrete manufacturing and process automation which permits the transmission of standard and safety-related data on a single bus cable.
  • The PROFl energy profile enables the design of systems with controllers such as PLC'S that can signal equipment referred to as energy consuming units ECUS to begin a sleep mode at the start of a planned pause or unplanned work stoppage. 
  • Software in each ECU determines how the sleep mode is implemented. When the sleep mode is over the controllers signal the ECUS to resume operation. 
  • The use of PROFl energy configurations enables substantial energy savings without a reduction in productivity.

Conclusion

PLC communication systems form the foundation of modern industrial automation by enabling reliable and efficient data exchange between various devices and control systems. Technologies such as PROFIBUS DP, PROFINET, Industrial Ethernet, and IO-Link provide scalable and high-performance solutions for diverse industrial applications.

Understanding these communication methods, network structures, and device types allows engineers and technicians to design robust, flexible, and future-ready automation systems. With continuous advancements in industrial networking, mastering these concepts is essential for anyone working in automation and control engineering.

About the Author – Insight Control System

  • Insight Control System provides technical education and practical guidance in the fields of Building Automation Systems (BAS), HVAC controls, PLC programming, sensors, and industrial automation. 
  • The content published on this platform is based on field experience, technical documentation analysis, and real-world system applications.
  • Our objective is to simplify complex automation concepts for engineers, technicians, students, and industry professionals worldwide.
  • Every article is carefully structured to deliver accurate technical knowledge, practical insights, and industry-relevant information aligned with modern automation standards.
  • We continuously update our content to reflect current technologies, control strategies, and best engineering practices.
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