19 July 2007

Industrial Ethernet for open automation networks

By Jim Pinto

Ethernet usage has increased in the automation environment, and the proliferation of Ethernet-ready products for device-level networks continues. For industrial networking, the big advantage is the ability to connect everything—from the plant floor to the corporate boardroom—on one network, providing access to data throughout the enterprise from anywhere around the world.

When first adopted in the mid-1980s as the IEEE 802.3 standard, Ethernet was considered unsuitable for automation purportedly because it was (and it still is) “nondeterministic.” The fundamental characteristic is the average delivery speed degrades in a non-linear way with loading. Device response times can not be guaranteed because of data collisions and delays in retransmitting data.

Ethernet’s “nondeterminism” has been vastly reduced by advanced switching technologies that allow multiple devices simultaneously to transmit and receive data over multiple network loops. Fast switches can quickly swap network lines, thereby responding very rapidly to anomalies such as missed communications, power failures, and device failures.

Adding some intelligence—namely, software or firmware—to the switch improves quality of service and adds queue management capabilities. By assigning a priority to time-sensitive data, intelligent Ethernet switches can elevate that traffic above lower-priority data. This ensures high-priority traffic always traverses the network even if the network becomes congested.

Ethernet now runs over shielded and unshielded twisted pair copper, coaxial, and EMI-resistant fiber-optic cable. Operating speeds across even conventional wire cabling have increased from 10 Mbps to 100 Mbps. Automatic switches that negotiate 10/100 Mbps are common, optimizing speed and service, as well as allowing a mix of 10 Mbps and 100 Mbps devices on the same network. The higher speed reduces the probability of data collisions and lost data, and delays are no longer an issue. For most factory automation applications, 100 Mbps Ethernet is deterministic enough.

Clearly, automation and control products must have Ethernet connectivity. The real question is whether devices should connect directly on Ethernet as nodes, or indirectly via one of the entrenched device networks.

Today, several industrial networks are in use, with different protocols—Modbus, Profibus, ControlNet/DeviceNet, Foundation Fieldbus, CANopen, etc. These are all supposedly “open” but nevertheless different because they serve different purposes. Ethernet is the link-layer protocol, with one of the above protocols as the application-layer. This is where commonality ends. Without a defined control networking standard, the different application protocols are simply bundled on top of TCP to complete their tasks.

My fundamental point is Ethernet TCP/IP on its own does not guarantee that two devices can talk to each other. Because they are incompatible at the application layer, they cannot communicate. To make this happen, a common data model, such as IEEE 1451, is needed that allows the porting of application-layer software from one platform to another.

The clear advantages have brought many automation enhancements to Ethernet standards. There is no longer any reason why Ethernet cannot help to build deterministic, open, and inexpensive automation network solutions.

The traditional industrial automation majors are clearly on the Ethernet bandwagon. For example, Rockwell Automation, Siemens, and others claim up to 70-80% of their automation products are Ethernet-enabled.

Related links:

• Industrial Ethernet for Automation Networks:
  http://www.automation.com/sitepages/pid2662.php
• The case for Industrial Ethernet:
  http://ethernet.industrial-networking.com/articles/articledisplay.asp?id=49
• Industrial Ethernet: Wiring the Enterprise:
  http://www.autofieldguide.com/articles/080307.html

Behind the byline

Jim Pinto is an industry analyst and founder of Action Instruments. You can e-mail him at jim@jimpinto.com or view his writings at www.JimPinto.com. Read the Table of Contents of his book, Pinto’s Points, at www.jimpinto.com/writings/points.html.