According to IMS Research fieldbus–based communications systems accounted for 75% of new industrial automation network connections in 2011, three times more than those using industrial Ethernet. IMS does, however, say Ethernet is growing faster and will become the dominant industrial networking technology within 10–15 years. The IMS study labels all non-Ethernet industrial networks as fieldbus, but for this article we'll use terminology more common to industrial automation. We'll define fieldbus networks as those used in process applications to connect instruments and analyzers to the main control system, typically a distributed control system (DCS). Leading examples are Foundation H1, HART and Profibus-PA.
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We'll identify device-level networks as those that connect discrete devices such as sensors, switches and motor starters to controllers, most typically PLCs and PACs. Some of the most popular are AS-i , CANopen, DeviceNet, IO-Link, Modbus and Profibus-DP.
Despite the Ethernet onslaught, device and fieldbus networks are still widely used in industrial applications. This article will explain why, using real-world applications, these networks are preferred over Ethernet in particular applications. We'll also gaze into the future, and speculate on the future of device, fieldbus and industrial Ethernet networks.
Simpler and Cheaper
When it comes to connecting field-level equipment — such as transmitters, control valves, motors, proximity sensors, encoders and similar monitoring and control devices — to control systems, device and fieldbus networks are often the simplest and cheapest way to go.
"Process controllers, temperature controllers, small ac drives and similar equipment typically have built in RS-232/RS485 ports," explains Paul Johnson, senior electrical engineer at CMD Corp., Appleton, Wis., which manufactures converting equipment for blown film (Figure 1). "Since most HMIs and PLCs have the same ports, they can support general-purpose protocols such as Modbus or DF1. We choose Modbus to save money. Ethernet support in many cases is an added expense. In some cases, Ethernet doesn't meet application requirements. The issues include too high a bandwidth, static noise, and its inability to interface to legacy equipment."
Carl Henning, deputy director at PI North America, the association for both Profibus and Profinet, agrees. "It makes no sense to put an Ethernet connection in a proximity switch or any other simple sensor or actuator," he maintains.
Ethernet can get complicated, too. "Ethernet typically calls for external hardware, architecture design and IT department involvement that industrial control engineers want to avoid if possible," says Bryan Sisler, product manager at ABB Low Voltage Drives.
Keeping the IT department out of the picture is easier with device and fieldbus networks. "Modbus can be handled by currently embedded protocols, no additional hardware is required, and speeds are adequate to handle most system requirements," Sisler says. "CANopen is simple to connect and use, is really open, PC-friendly, and great for low node-count systems. DeviceNet provides determinism, plug-and-play system components, and a hardy transceiver that can survive transients that Ethernet transceivers cannot." These, adds Sisler, are all reasons to avoid the expense, time consumption and complications brought on by more sophisticated networks, "especially those that might require the involvement and training of a new group of non-engineering folks."
Mallard Creek Polymers, Charlotte, N.C., a maker of latex products, also uses Modbus. "Device networks tend to be preferred in many cases due to their broad installation base, determinism and familiarity," says engineer Matt Bothe, who explains that MCP uses Modbus to communicate with several smart, mass flowmeters in order to extract flow and totalized measurements.
"Device networks generally possess advantages over analog/discrete systems, namely far less wire, ease of calibration, access to more data, ease of troubleshooting, and ease of installation," Bothe adds.
Determinism and Distance
One difference between device and fieldbus networks and Ethernet is determinism. Proponents argue that device and fieldbus networks guarantee a response time, while Ethernet can't, and this can be vitally important in certain applications.
"For position-dependent operations (Figure 2), deterministic behavior is a must," states Nick Hunt, manager of automotive technology and support at robot builder ABB Robotics. "For example, when a dispenser is told to turn on exactly one second before the robot arrives on point, it needs to be extremely consistent. We can even live with the fact that one second actually turns out to be 1.32 seconds. If it's consistent, it easily can be tuned in by including the latency in the robot controller's I/O config file."
And consistency seems to be the key. Ethernet, as fast as it is, isn't deterministic, so it can't provide the consistency needed for factory automation when timing is critical. "Speed is part of it, but speed really doesn't address the determinism issue, only the packet transfer rate," Hunt notes. "Determinism is addressed by the way collisions are handled, which they're not with Ethernet." Hunt goes on to say the raw speed of Ethernet can overcome most of the determinism concerns, but not all.
Another issue is distance. Wired Ethernet is limited to about 100 m, but device and fieldbus network distances can be much longer.