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Applying Wireless to Ethernet/IP Industrial Networks

Ethernet has matured into a standard communication protocol for industrial networks. Rockwell Automation's implementation - Ethernet IP - has become widely popular, in part, because economical infrastructure components are now readily available to users. Ease of configuration and operation, high data throughput, straightforward connectivity and ability to merge the factory floor with the corporate database have driven the Ethernet IP revolution in industry. In fact, its very success created a strong need to extend Ethernet IP networks beyond the ability to hardwire. Wireless Ethernet IP became an urgent requirement.

In addition to the elimination of costly wiring, wireless Ethernet has become standard in many work places because it provides a high degree of convenience and mobility. Wireless PCMCIA cards, access points, switches and hubs are now commonplace in homes and offices and are relatively inexpensive. The industrial environment requires more, however, and specialized Ethernet radio modems have been developed to meet the industrial need. These modems are physically hardened, have extended operating temperatures, allow the use of a wide variety of antennas and cables and have much greater range than commercial wireless products.

Wireless Ethernet does have some limitations. Operating latency and the need for message repeats in noisy environments sometimes exclude wireless Ethernet modems from consideration in deterministic applications. It is important to understand the responsibilities of the Ethernet/IP network or segment to determine if wireless is a viable option.

Generally, wireless is quite appropriate in applications where the network is used for data collection, equipment maintenance or non-critical (SCADA) control. The network is responsible for data exchange, but not for relaying time critical commands that, if delayed, could result in safety concerns or serious process disruption. Wireless is commonly integrated where PLCs directly control the local process, and need to be economically connected to the plant network. The network is used to transmit explicit messaging, data transfers that are acknowledged between the controllers. Explicit messaging ensures that data packets are reliably transmitted and resent if necessary.

Diagram 1: Example of a wireless Ethernet/IP network using explicit messaging. The network supports PLC programming, data collection and SCADA functions.

Ethernet/IP also supports direct I/O communication serving the role of a fieldbus over Ethernet cabling. In this mode, implicit messaging is used to directly communicate from the PLC program to the I/O device via the Ethernet network. The Ethernet network effectively serves as the communication infrastructure for distributed I/O.

Because Ethernet was not intended to be deterministic (having guaranteed response times), Rockwell Automation has established network design recommendations to achieve a high degree of deterministic performance. Following these guidelines essentially ensure that the Ethernet segment is as fast as possible (e.g. Full Duplex 100Base-T) and that unnecessary network traffic is kept to a minimum.

Wireless presents two concerns for use in deterministic I/O systems: data transmission rate and latency. First, wireless Ethernet systems cannot achieve the throughput of a Full Duplex 100Base-T segment. Emerging wireless technologies may eventually address this, but they are unproven at this time especially in industrial environments. Secondly, industrial wireless systems must contend with interference and retransmit packets when necessary. Error correction ensures that data packets are successfully received, but causes variability in message timing which negates high degree determinism. Direct sequence RF (e.g. IEEE 802.11b) has the best chance of predictable message timing, but direct sequence is not always ideal for factory floor communications due to interference susceptibility. Frequency hopping RF offers the highest level of interference immunity, but can have substantial latency and packet delay times.

Wireless Ethernet may be an ideal communication method for I/O if the system function and performance expectations are carefully considered. For example, if the I/O system is for data acquisition for machine monitoring purposes, then true determinism may not be needed and wireless quite appropriate. Similarly if the I/O is used for non-critical control, wireless may be acceptable. However if I/O timing is critical or safety mechanisms are dependent, then wireless is inappropriate.

Diagram 2: Example of a wireless Ethernet/IP network using implicit messaging for non-critical data acquisition and control.

A fundamental trade-off in any wireless modem is range and data rate. Data-Linc Group has modems that offer very long range (25+ miles with standard antennas) at modest data rates (100 kbps) or shorter range (2- 5 miles, depending on antenna) at high data rates (1 to 6 mbps). The SRM family of long-range, frequency hopping modems provide an exceptionally high level of noise/interference immunity in all operating conditions. The FastLinc 810E (Access point) and 800C (PCMCIA card) have maximum 802.11b data throughputs with typically twice the range of other products of its type. The FastLinc 810E can also be configured as a wireless bridge. All Data-Linc modems are packaged in heavy gauge steel, have extended operating temperatures and can be connected to a wide variety of antennas and cable lengths for use in challenging factory floor or severe outdoor operating conditions.

Data-Linc has extensive expertise in Rockwell Automation PLCs, HMIs, Drives and Ethernet IP. We provide both extensive pre- and post-sale technical support and commonly pre-configure modems for our customers. Data-Linc started as a Rockwell Pyramid Partner and was invited by Rockwell to become an Encompass Partner. Call 425-882-2206 to learn more about industrial wireless applications and product capabilities.

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