Communication Protocols for Industrial Automation

Three common protocols used in automation are serial fieldbus-based, Ethernet-based, and wireless. To perform better than the standard serial fieldbus and Ethernet communication protocols, some companies have developed some custom protocols for industrial automation purpose. Historically, industrial automation widely used serial fieldbus-based communication protocols in its earlier times. Although these serial fieldbus-based protocols have been used for a very long time, they are still frequently used these days because of the long lifecycle of industrial automation as well as because big companies are behind these serial fieldbus-based protocols. A data from HMS in 2017 shows that 42 percent of the market share of the industrial automation protocols are serial fieldbus-based. These include PROFIBUS DP, Modbus-RTU, CC-Link, CANopen, DeviceNet, and others.

However, due to the higher speed as well as other advantages of Ethernet compared to serial fieldbus, recently the industrial automation tends to use more the Ethernet-based communication protocols. According to the same data from HMS in 2017, 53 percent of the market share of the industrial automation protocols are Ethernet-based, with much higher growth than the serial fieldbus-based protocols. These Ethernet-based protocols include EtherNet/IP, PROFINET, EtherCAT, Modbus-TCP, POWERLINK, and others. Furthermore, only 6 percent of the market share is wireless protocols, including WLAN (Wifi), Bluetooth, and others.

Besides higher speed compared to serial fieldbus-based protocols, other advantages of Ethernet-based communication protocols include the following:

• Option to use optical fiber cable for longer distance.
• Ability to accommodate more nodes/addresses, i.e. the number of devices to be connected in the network.
• Potential to use peer-to-peer architectures as opposed to server-client (master-slave) architectures.

The following is a brief description about some commonly used serial fieldbus-based and Ethernet-based industrial automation protocols.

Serial fieldbus-based industrial automation protocols

PROFIBUS is the most successful, i.e. most installed, serial fieldbus protocol. Its speed is up to 12 Mbps and supports up to 126 nodes. It was first developed by a German research project and subsequently refined by Siemens in 1994. There are two variants of PROFIBUS, namely PROFIBUS DP and PROFIBUS PA. They are basically the same, only different in the physical layer. PROFIBUS DP typically runs on RS485 whereas PROFIBUS PA runs on MBP. Accordingly, PROFIBUS DP is faster but PROFIBUS PA is safer in hazardous environment.

Modbus is easily run on RS232 or RS485 with speed up to 115K baud and supports up to 247 nodes. It was developed by Modicon (now Schneider Electric) in 1979.

CC-Link is a serial fieldbus protocol running on RS485 with speed up to 10 Mbps and supports up to 64 nodes. It was originally developed by Mitsubishi Electric Corporation in 1997 and popular in Japan and Asia.

CAN (Control Area Network) is a high integrity serial fieldbus protocol, which was originally created for automotive purpose by Bosch GmbH in 1980, but then widely used in industrial automation. While the other, aformentioned serial protocols are not working at the physical layer, CAN is a physical-layer protocol. It means that CAN needs its own specific hardware to be implemented.

CANopen, developed by CiA, and DeviceNet, developed by Allen-Bradley (now Rockwell Automation), are higher level protocols built on top of CAN. CANopen supports up to 127 nodes whereas DeviceNet supports up to 64 nodes.

All of the above protocols do not reach milli-second cycle.

Ethernet-based industrial automation protocols

Since the standard TCP/IP or UDP/IP Ethernet communication protocols are non-deterministic with typical cycle times greater than 1000 milliseconds, some industrial Ethernet-based protocols have been introduced at various layers above the Ethernet physical layer.

EtherNet/IP was originally developed by Rockwell Automation and subsequently managend by ODVA. It is an application-layer protocol, working on top of TCP/IP protocol. It can be used on peer-to-peer architecture with unlimited number of nodes.

PROFINET was developed by Siemens. It is available in three main variants (classes): 1) PROFINET Class A with cycle time around 100 milliseconds, 2) PROFINET Class B, also called PROFINET Real-Time (PROFINET RT), with cycle time around 10 milliseconds, and 3) PROFINET Class C, also called PROFINET Isochronous Real-Time (PROFINET IRT), with cycle time to less than 1 millisecond. PROFINET Class A is good for parameter data and cyclic I/O, PROFINET RT is good for PLCs, and PROFINET IRT is good for motion applications. Branch and star are common topologies used for PROFINET.

EtherCAT was developed by Beckhoff. It is a MAC-layer protocol. It can have 65,535 nodes. It performs much faster than a standard Ethernet protocol because it resides at the MAC-layer as well as because of its data transfer scheme.

SERCOS III is the third generation of Serial Real-time Communication System (SERCOS). It can have 511 nodes. Its working principle is quite similar to EtherCAT. Hence, it is a MAC-layer protocol. Its cycle time is similar to EtherCAT and PROFINET IRT. It supports ring topology which introduces redundancy, as well as line topology. It is mostly used in servo applications.

CC-Link IE was developed by Mitsubishi Electric Corporation. It is a MAC-layer protocol. It has two variants: CC-Link IE Control, which is intended for controller-to-controller communications with a maximum of 120 nodes, and CC-Link IE Field, which is intended for I/O communications and motion control applications with a maximum of 254 nodes. CC-Link only supports ring topology which introduces redundancy.

Powerlink was developed by B&R in 2001. Its cycle time is 100 microseconds. With Gigabit Ethernet, its speed is 1000 Mbps. To increase the performance, it makes some modification at the data link layer. It uses the same device description files as CANopen; hence it describes itself as “CANopen over Ethernet”. It can have up to 240 nodes. It supports almost any Ethernet-based topology, including star, tree, daisy chain, and ring topologies.

Modbus TCP was developed by Schneider Electric. It is an extension of the serial Modbus. It is simple to be implemented on the standard Ethernet network, but does not guarantee real-time, determinism, and precise synchronization. Its performance is a strong function of the underlying TCP/UDP/IP network implementation. It allows any Ethernet-based topology.