Ethernet, in its many versions, continues to spread as the network of choice for industrial applications.
Leslie Langnau, Managing Editor
Industrial Ethernet-based networks are almost 40% of all industrial communication networks, and growing, outpacing many older fieldbus type networks. Drivers behind this growth include the ubiquity of Ethernet, it’s an established standard protocol, it has widespread support, is easy to obtain, and offers economies of scale, use and familiarity.
Six versions of Industrial Ethernet are CC-Link Industrial Ethernet, EtherCAT, EtherNet/IP, Ethernet Powerlink, Profinet IRT, and Sercos III. Each offers different performance parameters.
Data transmission speed and data reliability are probably the most important capabilities in an industrial network. Each network takes a different approach to deliver data within a given time frame, so understanding that approach will factor into your choice.
The hardware used in most Ethernet based networks is the same; network interface cards (NICs) and CAT5 Ethernet cables are quite capable of delivering data quickly and reliably. The key will be the real-time protocol used by the specific network. A typical protocol like TCP/UDP/IP, for example, which would ride over the Ethernet layers, does not deliver data at the speeds often needed by machine tools, but is perfectly adequate for other applications.
Thus, many industrial networks use one of three main approaches to deliver determinism using an Ethernet infrastructure.
The first approach is standard software and standard Ethernet. This approach uses TCP/IP layers with real-time mechanisms added to the top layer.
The second approach is “open” software with standard Ethernet. (Check with the controlling organization about how open is open.) The Ethernet protocol really covers just the cabling and how packets of data are arranged and transmitted through the cabling. With the open software and standard Ethernet approach, software protocols are implemented over the standard Ethernet protocol. The determinism comes from often proprietary portions of the software that schedules transmission of the data packets in specific times or setups. This software often resides at what is known as OSI Layer 3 and 4.
The third approach is open software and modified Ethernet. This approach often results in a new standard for Ethernet, such as IEEE 802.xxxxx. In some cases, this approach demands new hardware to execute the needed capabilities. The open software is free and available in a public domain.
Now, let’s look at specific details of each network.
CC-Link Industrial Ethernet
CC-Link of the CC-Link Partner Association (CLPA) is an open protocol used by many manufacturers. There are a number of variations for specific application needs: CC-Link/LT, CC-Link Safety, and CC-Link Industrial Ethernet (IE) Control and Field. Most versions offer speeds to 10 Mbps at 100 m.
CC-Link IE Field supports the demands of advanced automation. It leverages Gigabit Ethernet, so it can handle IIoT data quantities and speeds. The network links field-level devices to controllers, connects controllers to other controllers and provides deterministic communications without the use of switches. Users do not need detailed knowledge of Ethernet to use. It is a token-based communication approach, which means each device gets control of the network for a specific period of time.
The EtherCAT protocol was developed by Beckhoff Automation. The EtherCAT Technology Group (ETG) now has responsibility and control over the development of this protocol.
It takes an open software and modified Ethernet protocol approach. EtherCAT delivers deterministic response; it is able to process 1,000 I/O points in 32.5 μs, or data from 100 axes in 125 μs.
It uses a bus master (central controller) in a ring formation of connected devices. While EtherCAT uses the telegram structure of Ethernet, only one telegram is sent to all connected stations and slaves. The telegrams, or packets are divided into real-time and general data sections. As a packet travels around the ring, relevant output data are extracted by connected devices or input data are inserted into the packet. Very large data packets can be distributed across multiple EtherCAT frames.
EtherCAT is compatible with most drives and motion systems, regardless of manufacturer. More than 500 drive vendors have adopted EtherCAT compatibility as one of their network offerings.
Ethernet/IP was developed by Allen-Bradley (Rockwell Automation) and then turned over to the Open DeviceNet Vendors Association (ODVA). It follows the standard software and standard Ethernet approach.
EtherNet/IP is an application-layer protocol on top of TCP/IP. The Common Industrial Protocol (CIP) software is a part of EtherNet/IP. The CIP Sync code is used to distribute IEEE 1588 compliant time information throughout the network. In this manner, EtherNet/IP maintains 100% compatibility with Ethernet yet still delivers fast data update times and deterministic transmission.
Communication with devices, or nodes, is handled by CIP over the TCP connection. Multiple CIP connections can be handled over one TCP connection.
Through switches, EtherNet/IP can support an almost unlimited number of nodes. This protocol uses a producer-consumer services model for efficient slave peer-to-peer communications.
Ethernet/IP is time-based, which means control commands are received by the field stations in time. This method is how this protocol achieves determinism. Real-time delivery is safeguarded by three standards: UDP, Quality of Service (prioritization), and IEEE1588. Accessible bandwidth is limited to avoid contention and latency.
B&R developed ETHERNET Powerlink to provide standard Ethernet with real-time properties for motion applications. Now, the ETHERNET Powerlink Standardization Group (EPSG) manages this protocol. It uses the open software and standard Ethernet approach.
ETHERNET Powerlink is a cyclical protocol that organizes access to a network and the data synchronization of the devices. Based on a more traditional version of Ethernet, Ethernet Powerlink must work around the Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) method of data packet transmission. CSMA/CD does not deliver determinism. If two packets transmit from different nodes at the same time, they can “collide.” scrambling data. When CAMA/CD detects such a collision, it picks a random time to re-transmit one of the packets through the network. The randomness of the re-transmit time eliminates any determinism.
To solve this issue, Ethernet Powerlink imposes a time slot for critical data, which eliminates any possibility of collision. A software-based master controls the timing on the network and the master authorizes individual nodes to send data. The master (controller) polls the slaves (drives) within a specific portion of the communication cycle, known as the isochronous phase. Asynchronous data traffic can transmit over the remaining cycle time.
PROFINET comes in two flavors; PROFINET RT for soft real-time requirements and PROFINET IRT (IRT = isochronous real time) for what is referred to as “hard real-time” performance. Siemens developed this protocol with members of the Profibus organization. Profinet IRT is an open software and modified Ethernet approach.
PROFINET uses the producer/consumer model with additional protocols and services. Time critical data travel in Ethernet frames with virtual local area network prioritization. Other data, such as diagnostics and configuration data, are sent through UDP/IP. This arrangement means Profinet can offer cycle times of around 10 ms for I/O applications.
Profitnet IRT protocol uses a time multiplex mode based on managed, hardware-synchronized switches. It also uses a time slot mechanism, which works like this: a certain bandwidth is reserved for the real-time data traffic, and the remaining bandwidth is available for non-critical data transmissions. The nodes are connected to special switches rather than standard Ethernet switches. These special integrated switches use a special ASIC to control two or four ports at a data rate of 100 Mbps.
PROFINET is widely used by Siemens and GE, since it is embedded in their controllers and equipment.
SERCOS III (Serial Realtime Communication System) is an open protocol for high-speed serial communication of standard closed-loop realtime data over Industrial Ethernet cable. SERCOS International manages this protocol. It uses the open software and modified Ethernet approach.
SERCOS III uses 100 Mbps Ethernet and the Ethernet telegram. Determinism comes from the time slot approach in which bandwidth is reserved for the isochronous (real-time channel) and asynchronous (IP channel) data traffic.
SERCOS-III works without hubs or switches. Each station has an integrated ASIC or FPGA with two communication ports. This protocol can be arranged as either a line or ring topology.
Specific hardware is needed for the slaves on this network, but it is possible to use software for the network master.
It enables rich I/O communication and efficient transmission of all conventional protocols over the same Ethernet network—in parallel—with SERCOS real time communication. It has cycles times of 31.25 usec. It can support 511 slave devices in each network, with multiple networks possible in an installation.
CC-Link Partner Association
EtherCAT Technology Group
ETHERNET Powerlink Standardization Group