What is EtherNet/IP? Acromag White Paper
Getting Started with EtherNet/IP
This article will go over the operation of EtherNet/IP™ (EtherNet Industrial Protocol) as it relates to Acromag Series 900EN-60xx, XTxxx2-xxx, and NT expandable Series remote I/O modules. You can download a copy of the EtherNet/IP Standard, from the Open DeviceNet Vendor Association (ODVA) web site for Ethernet/IP.
Acromag also manufactures a line of I/O modules that support Modbus TCP/IP and EtherNet/IP. Get the latest information about these and other Acromag products on our website or by calling our Inside Sales Department at (248) 295-0880.
Table of Contents
- About EtherNet/IP
- The Open Systems Interconnect (OSI) Network Model
EtherNet/IP is the result of a joint effort between ODVA, ControlNet International (CI), and the Industrial Ethernet Association (IEA). Their goal was producing a network protocol to address the high demand for using the widely popular Ethernet network in control applications. They first presented it in March of 2000.
In short; EtherNet/IP (Ethernet Industrial Protocol) is traditional Ethernet, combined with an industrial application layer protocol, targeted to industrial automation. This application layer protocol is the Control and Information Protocol (CIP™).
IEEE 802.3 Ethernet is traditionally an office networking protocol that has gained universal acceptance world-wide. It’s an open standard supported by many manufacturers and its infrastructure equipment is widely available and largely installed. The Ethernet TCP/IP protocol suite is also found everywhere and it’s the foundation for access to the World Wide Web. Since many devices already support Ethernet I/O, it is only natural to augment it for use in industrial applications. EtherNet/IP was created to overcome the shortcomings of traditional IEEE 802.3 Ethernet. (As applied to the industrial automation world.)
Widely used in industrial environments for decades, the CIP provides both real-time and informational message structures. It’s designed to be “wire-independent,” in that it can work with any data-link and physical layer. CIP is freely available to anyone, easy to understand, and widely supported by many manufacturers of industrial equipment. As a result, it is a natural candidate for use in building other industrial communication standards. CIP was adopted after being merged with the popular CAN protocol in 1994; forming DeviceNet. ControlNet was the next protocol to adopt CIP in 1997. ControlNet has higher speeds (up to 5MB). Consequently, ControlNet is considered more deterministic than DeviceNet. Additionally, it extends the range of the bus, up to several kilometers, with the use of repeaters.
EtherNet/IP Merged IEEE 802.3 Ethernet with the CIP
Next, EtherNet/IP merged traditional IEEE 802.3 Ethernet with the CIP as its application layer to build an even more powerful industrial communication standard. EtherNet/IP shares the same physical and data link layers of traditional IEEE 802.3 Ethernet and uses the same TCP/IP suite of protocols. This makes it fully compatible with existing Ethernet hardware, such as cables, connectors, network interface cards, hubs, and switches. Further, EtherNet/IP uses the same application layer protocol used by both DeviceNet and ControlNet. Thus allowing these protocols to share common device profiles and object libraries. Additionally, it helps to make these types of devices interoperable on the same network.
There are four reasons why EtherNet/IP is considered an open network standard:
- The physical and data link layers use standard IEEE 802.3 Ethernet
- The network layer uses the TCP/IP suite of protocols
- Support by the following independent networking organizations:
- ControlNet International (CI)
- Industrial Ethernet Organization (IEA)
- Open DeviceNet Vendor Association (ODVA)
- Industrial Automation Open Network Alliance (IAONA)
- EtherNet/IP technology is also available free of charge to developers and vendors.
Historically, traditional Ethernet was not considered a viable fieldbus for industrial control and I/O networks. This was because of two major shortcomings; inherent nondeterminism, and low durability. However, new technology properly applied has mostly resolved these issues. Ethernet equipment was originally designed for the office environment; not harsh industrial settings. However, many factory Ethernet installations can use this standard hardware without a problem; new industrial-rated connectors, shielded cables, and hardened switches and hubs can now help resolve this durability issue.
With respect to the non-deterministic behavior of Ethernet; determinism is used here to describe the ability of a network protocol to guarantee a packet is sent/received in a finite, predictable amount of time. Thus, for critical control applications, determinism is very important.
Carrier Sense Multiple Access with Collision Detect
Carrier Sense Multiple Access with Collision Detect (CSMA/CD) is the arbitration protocol for carrier transmission access on any Ethernet network. Any network device can try to send a data frame at any time. However, with CSMA/CD applied, each device will first sense whether the line is idle and available for use. If the line is available, the device will then begin to transmit its first frame. If another device tries to send a frame at approximately the same time, then a collision occurs; as a result, both frames will be discarded.
Each device then waits a random amount of time and retries its transmission until its frame is sent successfully. This channel-allocation method is inherently non-deterministic. This is because a device may only transmit when the wire is free; thus resulting in unpredictable wait times before data may be transmitted. Additionally, because of cable signaling delay, collisions are still possible once the device begins to transmit the data; consequently forcing additional retransmission/retry cycles.
More Deterministic Ethernet with Fast Ethernet Switches
Most control systems have defined time requirements for packet transmission (typically less than 100ms). Therefore, the potential for collisions and the CSMA/CD method of retransmission isn’t considered deterministic behavior. As a result, traditional Ethernet had problems being accepted for use in critical control applications. However, Ethernet can be made more deterministic by using fast Ethernet switches. These switches increase the bandwidth of large networks by sub-dividing them into several smaller networks, called collision domains. The switch also provides a direct connection from sender to receiver, so only the receiver receives the data; not the entire network.
Switches (AKA switching hubs) are intelligent network devices that connect distributed Ethernet nodes more efficiently. Each port of a switch forwards data to another port based on the MAC address contained in the received data packet/frame. Switches learn and store the MAC addresses of each device they’re connected to, as well as the associated port numbers. The port of a switch doesn’t require its own MAC address.
During retransmission of a received packet, the switch port will instead look like the originating device by having assumed its source address; and the Ethernet collision domain is said to terminate at the switch port. The switch effectively breaks the network into separate data links/collision domains; one at each switch port. The ability of switches to target a packet to a specific port (rather than forwarding it to all switch ports), also help eliminate collisions that make Ethernet non-deterministic.
As switches become less expensive, the tendency in critical industrial control applications is to connect one Ethernet device per switch port; effectively treating the switch device as the hub of a star network. In this manner, with only one network device connected per switch port, the switch can run full-duplex; with no chance of collisions. Thus, a 10/100 Ethernet switch effectively runs at 20/200 Mbps because it can transmit and receive at 10 or 100 Mbps simultaneously (full-duplex). Since there’s only one device connected to a port, there’s no chance of a collision. The higher transfer speed of full-duplex, coupled without the need for invoking CSMA/CD, produces a more deterministic mode of operation. This helps critical control applications to remain both predictable and on-time.
Unfortunately, broadcast traffic on a company network cannot be completely filtered by switches. This may cause additional collisions, reducing the determinism of a network connecting more than one device to a switch port. However, if the company network and the control and I/O network are separated; traffic will not be added to the control network and the company network’s determinism will increase. Further, if a bridge is used to separate the two networks, then the bridge may be configured to filter unnecessary traffic.
Advances in Ethernet Switches
Combining good network design with fast switches and bridges where necessary raises the determinism of a network. This makes EtherNet/IP more appealing. Other advances in Ethernet switches, such as; higher speeds, broadcast storm protection, virtual LAN support, SNMP, and priority messaging, further help increase the determinism of Ethernet networks. As Gigabit (Gbit), 10Gbit, and 100Gbit Ethernet enters the market, determinism will no longer be a concern.
With Ethernet as an open standard, numerous hardware and software vendors compete; resulting in low-cost, off-the-shelf products. As almost everyone knows what Ethernet is, it is also easier and more efficient to train people on Ethernet networks. Research funding for Ethernet far surpasses that of any other fieldbus. This further enables faster development and increasingly higher speeds.
The Open Systems Interconnect (OSI) Network Model
For a better understanding of the meaning “open standard,” and how EtherNet/IP is structured, we need to review the (OSI) Reference Model. The International Standards Organization developed this model and it was adopted in 1983 as a common reference for the development of data communication standards. It does not attempt to define an implementation; but rather serves as a structural aide to understanding “what must be done” and “what goes where.”
As can be seen in Table 1, in the pdf, in the traditional OSI model, as well as the simplified 5-layer TCP/IP standard (layers 5 & 6 suppressed); the functions of communication are divided into seven (or five) layers. Every layer handles precisely defined tasks. For example; Layer 1 of this model is the physical layer and defines the physical transmission characteristics. The data link layer is the 2nd layer and defines the bus access protocol. Layer 7 is the application layer and defines the application functions. (This layer defines how device data will be interpreted.)
By the OSI Model, we can infer that for two devices to be interoperable on the same network, they must have the same application-layer protocol. In the past, many network devices have used their own proprietary protocols, and this has hindered their interoperability. This further drove the need for adoption of an open network I/O solution; one that would allow devices from a variety of vendors to seamlessly work together. This drive for interoperability was a key reason for creating EtherNet/IP.
In the TCP/IP Standard Model, Ethernet handles the bottom two layers (1 & 2) of the seven-layer OSI stack; while TCP/IP handles the next two layers (3 & 4). The application layer lies above TCP, IP, and Ethernet, and is the layer of information that gives meaning to the transmitted data.
Although the Acromag 9xxEN-60xx modules are designed for EtherNet/IP, they also provide support for one additional socket of Modbus TCP/IP.
With Acromag 9xxEN-40xx Modbus TCP/IP modules, the application layer protocol is Modbus. That is, Modbus TCP/IP uses Ethernet media and TCP/IP to communicate using an application layer with the same register access method as Modbus RTU. Many manufacturers happen to support Modbus RTU and TCP/IP. Additionally, Modbus is widely understood and freely distributed, thus, Modbus TCP/IP is also considered an open standard.
Acromag’s 9xxEN-60xx modules are designed for EtherNet/IP. However, they also provide support for one additional socket of Modbus TCP/IP. With Acromag 9xxEN-60xx EtherNet/IP and XTxxx2-xxx modules, the application layer protocol is the CIP. The same application layer protocol is used by ControlNet and DeviceNet devices. By sharing the same application layer, these devices can be made interoperable on the same network.
EtherNet/IP is based on the TCP/IP protocol family and shares the same lower four layers of the OSI model common to all Ethernet devices. This makes it fully compatible with existing Ethernet hardware, such as cables, connectors, network interface cards, hubs, and switches. However, EtherNet/IP adds the Control and Information Protocol (CIP) as its application layer. Again, the same application layer protocol is used by both DeviceNet and ControlNet devices; making each of the device types interoperable on the same network. It further allows these protocols to share common device profiles and object libraries.
Learn more about Ethernet: How to Connect to an Ethernet Device for Communication Whitepaper
You will get more information about:
- Application Layer
- Object-Oriented Terminology
- Control & Information Protocol (CIP)
- CIP Encapsulation Message
- Connection Manager
- Transport Layer
- Transport Control Protocol (TCP)
- TCP Example
- User Datagram Protocol (UDP)
- Network Layer
- Internet Protocol (IP)
- Ethernet (MAC) Address
- Internet (IP) Address
- Address Resolution Protocol (ARP)
- Reverse Address Resolution Protocol (RARP)
- Data Link Layer
- Carrier Sense Multiple Access w/CD (CSMA/CD)
- Medium Access Control (MAC) Protocol
- Ethernet (MAC) Packet
- Electronic Data Sheet (EDS) File