Copyright © Romilly Bowden 1996, 1997.
HART (Highway Addressable Remote Transducer) is a digital communication protocol which operates on top of a conventional 4-to-20mA current loop signal from a measurement device - a transmitter - or to a valve positioner. HART uses a low-level frequency-shift-keyed sine-wave signal to convey the digital communication. This has an average value of zero, so does not affect the analogue signal. Because the analogue current loop is typically unbalanced (grounded on one side), and not impedance-matched, cable capacitance attenuates high-frequency signals. HART therefore uses relatively low frequencies (1200 and 2200 Hz) and this, in turn, restricts it to 1200 baud (equivalent to 1200 bits per second, with simple "1-bit-per-symbol" coding techniques). Similar modulation methods used to carry digital signals over telephone lines have somewhat more capacity, because they do not have to contend with the possible levels of cross-talk which can occur in the multi-pair cables commonly used in industrial process field wiring.
Fieldbus is a generic name given to fully-digital communication protocols for industrial measurement and control applications. Ideally, the cables used are more closely related to communication technology than to process instrumentation, but operation is also possible over conventional process field cabling, at least over limited distances (say up to 400 m.). The intention is that the same protocol should be usable, with appropriate communication speeds, for both process control and factory automation (machinery control). The IEC and the ISA (now, rather confusingly, called the International Society for Measurement and Control) have more or less coordinated their efforts to reach a worldwide international standard, which will be IEC 1158 and ISA S50.02. For process control, the significant features of the IEC/ISA Fieldbus are:
Notice that HART offers all these features except the first.
In Europe, CENELEC has approved a "General Purpose Field Communication System" EuroNorm EN50170 (best to say five-oh-one-seven-oh or five-oh-one-seventy, not fifty-one-seventy!). This is not a new fieldbus protocol, but a composite of three existing national standard protocols: Profibus from Germany (DIN19245), WorldFIP from France (NF C46602-46607) and P-Net from Denmark. EN50170 includes these three incompatible bus systems as separate parts of a single standard, leaving users to choose which one to implement in a particular system.
WorldFIP and Profibus (in its new "PA" variant) both include options to use the IEC1158 physical layer for 2-wire operation and intrinsic safety. (Profibus PA is not yet a part of EN50170, but will probably be accepted shortly.) P-Net runs on a proprietary version of RS-485 as a physical layer; this no doubt has its place, but can hardly be considered a "fieldbus" in the context of process control applications (see the features list above).
Many users have criticized EN50170, since it does not offer the single worldwide international fieldbus which they had been waiting for, and which the ongoing IEC and ISA work aims to produce. Yet its existence automatically restricts freedom of choice for some European users (public utilities, government contracts, etc.). In response to this concern, the British Standards Institution has adopted FOUNDATION Fieldbus ("FF") as a UK Draft for Development (DD238) and proposed it as a fourth option for EN50170, on the basis that it is nearest to the likely future IEC/ISA standard. FF uses the 2-wire IEC1158 physical layer and a subset of the latest proposals for the IEC1158 data link layer. Acceptance of FF into EN50170 will be voted on by European national standards committees in August this year. If FF is accepted, those users who are constrained to follow CENELEC standards will retain their freedom to choose between all the major fieldbus contenders, based on the functionality they require and their expectations for the future.
In judging the relative merits of HART and the various present and future "fieldbuses", you need to think about what they are to be used for. HART has typically been used for remote configuration, adjustment and diagnostics of smart field devices, and for this it is very adequate. Generally, and especially where fast response is important, it is not a good idea to use HART communication to read measurement values; the continued existence of the analogue signal is important in such cases. So is the continued compatibility with existing analogue connections on most control systems (SCADA and DCS). But in some applications, where high data rate is not required, HART has also been successful in its multidrop mode (up to 15 devices). In this mode, the analogue signal is not used, and all measurements are made using HART digital communication. Even when using the analogue signal for the measurement, you can gain a valuable extra degree of confidence in that measurement by continually checking the status of the field device by HART communication. (HART-compatible DCSs usually do this.)
In future, we can expect that field devices will become more complex, "smarter", and will hold more information about themselves and the process to which they are connected. A higher communication data rate will be important in handling this additional information without undue delays. Fieldbus (at 31.25 kbps) will provide this capability, and will also be ample for measurement of process pressures, temperatures and flowrates. The higher-speed variants (at 1Mbps and 2.5 Mbps) will be necessary for machinery control and in PLC applications where millisecond response times are required.
Field devices containing information about themselves - for example, their materials of construction, calibration, measurement range and application - offer a great step forward over earlier instruments. It becomes possible to keep plant records up-to-date and accurate simply by interrogating the field device itself. Validation of process operation is becoming increasingly important, both because some products demand it (pharmaceuticals, for example) and because it provides invaluable information in the later investigation of production problems. Software packages are now available which perform this record-keeping automatically, when connected to HART networks. For example, Applied System Technologies' "Cornerstone" and Fisher-Rosemount's "Asset Management Solutions" (AMS).
You don't have to wait for Fieldbus to get these benefits.