Applying Wireless To EtherNet/IP Pipeline Automation

Gary Enstad and Jim Ralston, ProSoft Technology®, Madison, WI
February 2011, Vol. 238 No. 2

Figure 1: EtherNet/IP Protocol Stack.

Buyer's Guide

The use of Ethernet for industrial networking is growing rapidly in process control and SCADA systems such as oil and gas pipeline applications. The Open Device Net Vendor Association (ODVA) EtherNet/IP network standard is gaining popularity as a preferred industrial protocol. Engineers are recognizing the significant advantages that Ethernet-enabled devices provide such as ease of connectivity, high performance and cost savings.

While EtherNet/IP has many advantages, cable installation is often expensive, and communications to remote sites may not be reliable or cost-effective. Wireless Ethernet technologies have emerged that can now reliably reduce network costs while improving efficiencies.

However, applying these technologies is not a simple matter because industrial Ethernet systems vary greatly in terms of bandwidth requirements, response times and data transmission characteristics. This article will only briefly provide brief excerpts from a much more detailed discussion by the authors about applying IEEE 802.11a/b/g and proprietary frequency hopping wireless technologies to EtherNet/IP based networks for pipeline and other industrial automation systems.

Ethernet Industrial Protocol (EtherNet/IP) is a network protocol defined by ODVA. As an open standard, vendors may implement EtherNet/IP communications in their devices without licensing fees. Many vendors have adopted EtherNet/IP including Rockwell Automation, who selected the protocol as one of three preferred networks on their popular Logix controllers (DeviceNet and ControlNet are the other two).

An important part of the EtherNet/IP standard is definition of Common Industrial Protocol (CIP) messaging. CIP defines the information packet with recognition that the message attributes will vary as applications do. Thus CIP message definition takes into account a wide range of applications including programming/diagnostics, data collection, PLC data exchange and I/O communications. EtherNet/IP uses the standard 7 layer OSI model for protocol definition as shown in Figure 1.

Implicit Vs. Explicit Messaging. CIP defines two different types of connections. The first type is Explicit CIP which uses TCP/IP for its communications protocol. Explicit messages are unscheduled and use a request/response communications procedure or client/server connection. Examples of Explicit: executing a MSG statement between PLCs, HMIs, device diagnostics and program uploads/downloads.

The second type of CIP is Implicit. Implicit uses UDP/IP for its communication mechanism. Implicit connections are time critical, are scheduled and use a requested packet interval (RPI) parameter to specify the rate at which data updates.

Implicit connections use UDP packets for produce/consume data over an Ethernet/IP network. The UDP packets are usually multicast if there is more than one consumer of the data. This multicast address is assigned by the Ethernet/IP interface and is unique for each produced tag. Multicast IP addresses are used to make the network more efficient. A producer of data can produce data for multiple consumers. By using multicast packets, many devices can receive or consume this packet without the producer having to send it to each individual consumer.