Tech Spotlight: PROFINET Protocol

Introduction

Profinet is an open Industrial Ethernet Standard, developed and maintained by Profibus & Profinet International (PI). It provides both wired and wireless connectivity and permits different industrial applications to use high-speed networks for device-to-device communication. The use of Profinet with the well-established Profibus network offers high system availability and simple extensibility, reduces commissioning, and decreases installation costs.

Communication Networks

There are two communication networks in Profinet, Wired, and Wireless.

• Wired: Profinet provides high-speed wired networking using fiber optic and copper cable with 10 Mbps and 100 Mbps speed for real-time control. Profinet uses one or a mix of various standard wired Network topologies like Star, Tree, Ring, and Line. A Star topology is used for small radius expansion and high device density applications such as single manufacturing machine. Using the Tree topology, several stars can be connected. A Ring topology can be used in systems that require high availability to protect against faulty network component. Line topology is used for extensive structure systems and can be implemented using a switch near the connecting terminal.
• Wireless: In the Industrial Internet of Things (IIOT) environment, it becomes imperative to use wireless connectivity for communication. Profinet supports wireless connection using Bluetooth and Industrial Wireless LAN (IWLAN) according to IEEE 802.15 and IEEE 802.11 standard with 2.4 GHz frequency. The profile specifies how to integrate wireless radio module with Profinet. The mobility and flexibility of wireless network infrastructure enable the use of mobile operating devices, saves cabling and installation cost.

Function Classes

There are two Profinet Function Classes: Profinet CBA (Component-based Automation) and Profinet I/O (Input/Output).

Profinet CBA: Profinet CBA is suitable for intelligent field devices with the controller and programmable functionalities in distributed industrial automation applications. Distributed intelligence provides a modular structure of plant and production lines to provide shorter response time. Profinet CBA uses Remote Procedure Call (RPC) for client/server communication. RPC is unable to link process at the object level. Distributed Component Object Model (DCOM) technologies from Microsoft overcome this limitation. the DCOM protocol defines how a client can locate, request, and receive data from the server. In Profinet CBA, disconnected DCOM objects are technological components described by the Profinet Component Description (PCD) file.

Profinet IO:  Profinet IO follows the provider/consumer model for data exchange. In general, there are three device types of Profinet IO:

• IO controller: Controller represents a scanner or PLC, responsible for handling field device activities such as configuration, alarm handling, and IP address assignment.
• IO Device: An assigned field device that connects remotely to the IO controller. It provides alarm information and diagnosis for protocol, process events, and equipment.
• IO Supervisor: A computer or programming device with diagnosis and commissioning facility which supports client function to inspect, maintain and remote control an IO device.

Profinet IO is a distributed I/O and is used to connect distributed field devices with a control system for data exchange via three communication performance levels:

• Non-Real Time (NRT):  Profinet transmits data packets using TCP/IP and UDP protocol and is applicable for non-time-critical processes.
• Real Time (RT): In industrial automation, read-write operation requires high-speed data transfer for optimized data exchange performance and alarms. Bypassing the time needed to process the TCP/IP stack helps to achieve this.
• Isochronous Real Time (IRT):  This communication channel is used for clock-synchronized communication with immediate response (< 1 millisecond) like motion control applications.

To configure the Profinet IO system, the controller manufacturer has to provide configuration tool. Profinet uses TCP/IP, UDP and XML IT standards to set, communicate and diagnose the field devices or machines. The distributed field devices are assigned to one or more controllers during configuration. The characteristics such as functions and properties of these field devices are described in a General Station Description (GSD) file based on Extensible Markup Language (XML) provided by the field device manufacturer.

Network Layers

Profinet is a Five-Layer Network Model adapted by deleting Session and Presentation layer from seven-layer OSI model.

Figure 1. Network Model

• Physical Layer: Profinet uses Fast Ethernet with 100 Mbps speed for data transmission. Fast Ethernet works on Full-Duplex mode.
• Data link Layer: This layer provides error-free data transmission between two users. Data is in the form of a frame, which contains a Medium Access Control (MAC) address. Profinet PI offers device manufacturers an OUI (Organizationally Unique Identifier) of MAC address.
• Network Layer: Its job is to establish communication between remote users by using their IP address. The IP address must be unique within one network. By using IP address, we can access the correct Profinet device. For network expansion, it is necessary to use a proprietary engineering tool from device manufacturer to provide a network address to Profinet devices.
• Transport Layer: The job of this layer is to ensure the connection. User Datagram Protocol (UDP) or Transmission Control Protocol (TCP) is used to implement the process of data exchange.
• Application Layer: This layer defines the Proxy mechanism which enables seamless integration between installed Profibus and Profinet.

Profinet Frame

Profinet uses Ethernet as a communication medium.  Below figure shows the data frame format of Ethernet.

Figure 2: Profinet Frame

• Preamble: The frame begins with a start bit called a preamble. It consists of 7 bytes of alternate bits. Ex. 1010101 and so on.
• Start Frame Delimiter (SFD): Consecutive ‘1s’ in series, indicates that a valid frame is about to begin. Ex: 10101011
• Destination Address: It shows receivers MAC (Media Access Control) address. The First bit represents, whether the address is unicast (0) or multicast (1).
• Source Address: It shows the MAC address of the Sender.
• Length / EtherType:  It shows the length of the data field and EtherType. EtherType ID for Profinet is 0x8892.
• Data: Data can be 1500 bytes but not less than 46 bytes.
• Frame Check Sequence (FCS): This field consists of checksum data used for Data integrity purpose.

Application Profiles

Application profiles are specifications for performance features, specific properties, and activities of systems that are developed jointly by users and manufacturers.

• PROFIsafe: Using a standard IO mechanism, this profile implements secure communication and provides functional safety. Functional safety means protection from hazards caused by incorrect functioning.
• PROFIdrive: It describes the standard interface between the control process and drivers. It is used for motion control applications where response time under 100 milliseconds is required.
• PROFIenergy: This technology addresses Energy Management. It helps to reduce utility bills in the manufacturing industry. Using PROFIenergy, the PLC automatically manages the energy usage through the existing automation network.

Integration with Profibus

Profinet provides seamless integration with the existing serial Fieldbus technology, like Profibus, without infrastructure modification.

Figure 3. Profibus and Profinet Integration

Figure 3 shows the transition from the Fieldbus to Profinet system. The Proxy (Gateway) enables connections between existing Profibus System and Profinet. The proxy provides a simple and high degree of transparent communication between field devices.

• For Profibus, the Proxy acts as a Profibus Master and coordinates the communication between Profibus nodes. the Proxy saves the complete functionality of lower-level Fieldbus.
• For Profinet, the Proxy acts as an Ethernet device, which represents one or more Profibus devices. The Proxy coordinates the Fieldbus specific data traffic and Ethernet data traffic.

Conclusion

Profinet is widely used in areas such as Manufacturing Automation, Building Automation, Process Automation, Power generation Industry, and Oil and Gas industry.