The New Game in Fieldbus

June 7, 2010
Fieldbus Reduced the Number of I/O Cards Needed at the Control Room, but It Still Requires Third-Party Power Supplies and Power Conditioners

While it doesn't obviate the need for wires altogether, Foundation fieldbus and other bus technologies do reduce the need for traditional rack-mounted I/O cards by moving the I/O function into the field devices themselves.

In a typical fieldbus topology, a number of these digitally communicating devices—whether speaking Foundation, Profibus, ASI-bus or any of a number of other protocols—can coexist on a single pair of wires, communicating not only process variable or status information, but a wealth of other secondary information as well. In the case of Foundation fieldbus, up to 16 devices can be daisy-chained along a single pair of wires, and the traditional I/O cards that would have been needed are replace by a rack-mounted H1 card to manage communications between the control system and the devices along that given segment.

But a funny thing happened on the way to the reduced wiring costs and better information access promised by fieldbus. While fieldbus reduced the number of I/O cards needed back at the control room or junction box, it still required third-party power supplies and power conditioners. And, if you wanted the ability to access fieldbus diagnostics directly in the control system, more third-party devices, software and integration work were needed. Further, installing fieldbus properly came with its own set of "gotchas." In the end, using fieldbus instead of point-to-point wiring did little to reduce the time and effort needed to engineer and implement I/O points.

HCD Meets Foundation

Enter Emerson's human-centered design (HCD) initiative, which took a hard look at Foundation fieldbus in the course of its DeltaV S-series development work, and found that substantial improvements could be made in how fieldbus gets done, according to Duncan Schleiss, Emerson vice president of platform strategy. "By applying the principles of HCD, we have changed the game in fieldbus again," Schleiss says.

Foundation fieldbus technology requires third-party power supplies and power conditioners, engineering, wiring and cabinet space. Also, to get at the bus diagnostics, you have to jump through OPC and RS485 hoops in order to get the information into the control system.

With DeltaV S-series and I/O on Demand, Emerson has integrated the power circuitry within the H1 card itself, explains Schleiss. This eliminates difficult segment power design, installation and troubleshooting tasks. The additional cabinet footprint associated with use of external segment power supplies is also eliminated. Third-party power conditioners and dedicated power supplies and all the engineering that went with them are no longer necessary. They have become a thing of the past.

Citing a recently completed project consisting of some 2,500 Foundation fieldbus segments, Emerson estimates that total project cost could have been reduced by some 7% using the new I/O on Demand approach. "Five thousand H1 power conditioners, 32 H1 power cabinets, along with their design and documentation, and all that factory and on-site wiring," lists Schleiss. "All gone!"

All This and Diagnostics, Too

"But we also integrated diagnostics," Schleiss says. "So now the maintenance staff can wait for the DeltaV H1 card to notify them of a problem on the physical layer, such as power consumption or a missing terminator. Easy! It's all integrated!"

The DeltaV digital automation system also works natively with other popular digital communication buses, such as Profibus DP and DeviceNet for integration of motor starters and drives, and AS-i bus for low-cost, simple installation of discrete devices, such as push-buttons, on/off valves and proximity sensors.

"The DeltaV system provides native support for configuring busses with no need for third-party configuration tools," Schleiss says. "And Ethernet I/O devices are easily connected through virtual I/O modules."

Further, fieldbus devices are auto-sensed when connected to the DeltaV system network and automatically added to the system configuration, Schleiss adds. "As a result, engineering and commissioning effort is dramatically reduced."