The Fieldbus Jungle -- Part II

By P. Hunter Vegas, Avid Solutions

This is Part II of a two-part article. Part I in the Q1 2011 issue detailed what fieldbuses can do compared with the claims made about them. You can read both parts now at www.industrialnetworking.net/fieldbus11q1.

Based on Part I of this article, you might wonder if fieldbuses are worth the effort. The answer is a resounding "Yes"—in the right application. It also can be an equally resounding "No" in the wrong situation.

This time we'll explore the major fieldbuses and address situations that favor and disfavor each. The high-speed network communications buses (Profibus-DP, EtherNet/IP, ControlNet and others) are not included because this article concentrates on the more common, low-level buses that communicate directly with the field devices.

Actuator-Sensor Interface (AS-i)

AS-i is among the simplest networks to implement. It is generally used to communicate with digital devices (solenoids, pushbuttons, on/off valves, etc.). It has some limited ability to transmit analog information, but rarely is used for that. It is a simple network, has a reasonably high speed (167 Kbps, <5 ms cycle time), and is insensitive to electrical interferences. The major limitation of the network is length. The total cable length of a network must be limited to 100 m. However, the recent introduction of low-cost network repeaters and power conditioners allow a single AS-i power supply to power multiple 100 m segments of the same network.

Advantage AS-i

AS-i favors any process that involves numerous digital devices in a relatively small area. A batch reactor is often a great application for AS-i because the reactor usually has a large number of on/off valves controlling the jacket and raw material charges in close proximity to the reactor. Because each valve has at least one solenoid and two limit switches, wiring savings can be significant.

AS-i is best used when the control system has an integrated AS-i card. It is possible to install an AS-i card in third-party remote gateway racks (via Profibus-DP, DeviceNet and others), but that arrangement makes engineering more difficult and slows valve response time.

It is good for non-classified, Class I Div 2, and Class I Div 1 areas. Recent introductions in AS-i wiring equipment make conduit routing simpler and more cost-effective, especially in electrically classified areas.

Network "tuners" also have been introduced recently to extend the network beyond 100 m by automatically optimizing the network capacitance and termination resistance. Its network cable has no particular topology requirements, so it can be branched as necessary to accommodate field equipment.

AS-i Profile files don't change often, so old and new equipment are generally interchangeable, I/O modules can be wired to pushbuttons and/or power lights to handle local operator stations, and the networks are rated to 8 A. This provides much more available power than most fieldbus networks. Configuration of a device is simple; just assign it a node number.

It's Not Always Right

AS-i is designed for on/off devices and is ill-suited for analog instrumentation. If the process equipment is scattered across a plant, the 100 m limit will prove too constraining even with repeaters. And due to the 100 m limit, you must know where the valves are going to be installed, often before the plant is built. If equipment moves, the network design must be rechecked.

AS-i valve heads cost more than standard on/off heads with limit switches. The valves tend to use low-power solenoids, so if the instrument air supply is poor, the solenoids could become a maintenance issue.

Like any network, getting the first valve or device to communicate can be a challenge. However, once that valve is established, configuration only involves hooking up an AS-i handheld to the device and giving it a node number. Checkouts are extremely quick.

Spotlight on DeviceNet

DeviceNet is based on the CIP protocol and typically is used for digital devices and motor drives. It is a deterministic, high-speed (500, 250 or 125 Kbps) network that also provides power on its multi-conductor cable for field devices. It is a more complex network and will be more difficult to set up than AS-i, but it handles a much larger variety of data. The allowable length of a DeviceNet network is speed-dependent, but the length limitations are not nearly as constraining as AS-i.

The best applications for DeviceNet are motor control centers (MCCs) or discrete manufacturing lines where a single bus cable can run along the system with short drops for each device. Cable distances usually are not a constraint, so knowing the precise equipment location in advance is not critical, and equipment can be moved without impacting the network significantly.

DeviceNet 120 Vac motor modules can be a good way to solve issues with NEC Arc Flash requirements. Running the 110 Vac motor control leads to a cabinet with the DeviceNet devices lets electricians make voltage checks on live motor systems without exposure to 480 Vac.

I/O modules can be wired to pushbuttons and/or power lights to handle local operator stations. This network is generally rated to 8 A and provides much more available power than most fieldbuses.

An Auto Recovery feature allows a DeviceNet device to be replaced and automatically downloaded with the proper configuration when it is put on the network. There are some limitations, but this feature can shorten system downtime—if the replacement device is an exact duplicate.