Hart was originally designed in the mid-1980s by a single company (Rosemount) to add digital communications to process field devices in analog process control loops. Such loops use standard analog 4-20 milliAmp (mA) signals for process control and communication between sensors and actuators that control process loops in the frequency range of 0 to 10 Hertz.
While millions of Hart devices are installed in process manufacturing today, most of them do not communicate digitally, except for occasional sessions with hand-held calibration/diagnostic units. Thus, the potential value of their ability to communicate digitally is most often lost because no device in the loop is listening to anything except the analog 4-20 mA signal.
After Hart was developed, imaginations in the process control world turned to the prospect of a fully digital fieldbus. This vision was also of a single protocol to eventually replace all analog 4-20 mA field devices. This digital fieldbus would consign technologies such as Hart to the dustbin of history. Fieldbus was designed to run over existing plant wiring at the breathtaking rate of 31.25 kilobits/second. But process fieldbus never became a single global standard. The process manufacturing world today contains both Foundation Fieldbus H1 and Profibus PA field devices, which share a common IEC 1158-2 physical layer, as defined by the International Electrotechnical Commission, but share very little else.
Fieldbus was meant to be a fully digital replacement for the analog transmission used by legacy process control systems. Fieldbus would use its greater bandwidth to enable applications such as alarming, field-based control, and measurement redundancy over digital field networks. Legacy technologies such as Hart could not perform these tasks and, some said, did not really merit the title of “process fieldbus,” which was fought over by the Foundation and Profibus consortiums. Hart was suitable only for the online interrogation of field devices in an analog loop.
Hart and Wireless
Enter wireless. The prospect of wireless field devices in the process industries has excited imaginations to a frenzy. While the total installed cost of process field devices will always include the “wet” process connection plus the cost of the device itself, eliminating the field wiring could be a huge gain for the process industries, and could change the economics of field measurement devices in ways that make device manufacturers salivate. But how best should such future wireless devices communicate?
The protocols employed by Foundation Fieldbus and Profibus-PA rely on their wired 31 kB physical layer. The ancient Hart protocol is far less demanding (some would say clunky) in that it expects to communicate at only 1,200 bits/second and can flourish in a single-master/single-slave configuration. The more advanced fieldbus protocols expect to share the 31 kB bus among multiple devices for multiple purposes. When transferred to the world of wireless, the archaic clunkiness of the older Hart becomes a virtue, while the more modern requirements of Foundation Fieldbus and Profibus PA are harder to satisfy without wire.
Today, the wireless field device is the dream product of process manufacturers. However, in order to develop such game-changing products, conserving battery power has become the paramount concern in product design. A clunky old communication protocol is not a liability if it enables a longer device service life and easier migration from the wired world of the 20th century to the wireless world of tomorrow. Hence, field device manufacturers are looking toward the ancient and venerable Hart protocol to be encapsulated and transported by modern wireless mesh networks. Hart may be an excellent fit for future wireless field devices that must conserve their battery power and operate reliably over the widely variable conditions encountered by field wireless networks.
The last has become first.
Harry Forbes, [email protected], is a senior analyst at ARC Advisory Group Inc., Dedham, Mass.
