[Engineering Feature]
Industrial-Strength Networking
The integration of Ethernet and wireless interfaces into microcontrollers makes networking even easier in industrial automation.
Every line of microcontrollers includes a model with at least one Ethernet interface. That's why it isn't surprising to find Ethernet up and down the industrial control pyramid (Fig. 1).
Ethernet has always been found in the upper reaches where management functions occur. But lately, it's been moving down into the fieldbus realm, which once was dominated by more than half a dozen high-speed serial interfaces like Modbus and Profibus.
Merging this plethora of communications systems into a single Ethernet standard would have been a laudable goal, though there's zero chance that will happen. Still, the list of Ethernet-based, industrial-control implementations is shorter than the list of fieldbus implementations (see the table).
Even the one attempt in 2000 to come up with a consolidated fieldbus standard resulted in the IEC fieldbus standard, 61158. Its multiple protocol sets included Foundation Fieldbus H1, ControlNet, Profibus, P-Net, Foundation Fieldbus HSE (High Speed Ethernet), Interbus, and WorldFIP.
Why are there so many Ethernet options? Reasons range from the need for compatibility and migration options for older technology to tradeoffs between using standard Ethernet implementations, or versions that incorporate extensions to the standard to implement real-time support. Yet cooperation may prevail when it comes to interchange and definition standards.
The Fieldbus Foundation, Profibus, and the OPC Foundation support the International Electrotechnical Commission (IEC) 61804-3 international standard on Electronic Device Description Language (EDDL). The ISA (Instrumentation, Systems, and Automation Society) SP104 committee will republish the IEC standard as an American National Standard Institute (ANSI) standard.
Approaches that extend Ethernet typically require custom Ethernet controllers. Then, however, they won't be suitable for off-the-shelf microcontrollers with on-chip Ethernet interfaces. That's because it's unlikely the real-time extensions could be implemented with anything short of an FPGA or ASIC.
Typical TCP/IP Ethernet implementations have latencies on the order of 1 to 2 ms for 100BaseT. Worse for real-time applications, Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection). This means one network node can step on the transmission of another. Ethernet recovers gracefully, but neither transmission succeeds and both must be re-sent.
This could have catastrophic results in a real-time system. Also, it's one reason why Ethernet often is mixed with other fieldbus implementations such as CAN, Modbus, and Profibus, which can provide isochronous, real-time support.
Some Ethernet-related standards tend to go across most of the industrial Ethernet implementations, like the IEEE-1588 time synchronization standard and TCP/IP, of course (see "Keeping Time Ethernet-Style" at www.electronicdesign.com, Drill Deeper 13807). Existing Power over Ethernet (PoE) standards are being employed as well.
Yet PoE doesn't always suit industrial requirements, which are usually more demanding when compared to office environments (see "Industrial Power Over Ethernet," p. 46). Also, 24 V is the industrial standard, while the existing PoE is the 48 V found in most communication systems.
Industrial Ethernet implementations that remain true to the Ethernet standard work with 802.11 Wi-Fi. But real-time enhancements are unlikely to propagate through the air with the desired results.
Complicating wireless industrial controls are new wireless standards such as 802.15.4 and ZigBee (see "Industrial 802.15.4, ZigBee, And Wi-Fi," p. 44). This discussion is best left to another day, but keep in mind that industrial Ethernet likely will be the backbone for wireless sensor and control networks.
The description of PROFINET applies only to the isochronous real-time (IRT) method of communication typically used in motion control. Standard switches and other Ethernet devices (including wireless) are usable with PROFINET real-time. For more details, download the PROFINET booklet at http://www.us.profibus.com/resources.aspx?pagetype=sue.
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