There are four leading types of protocols commonly used in process plants: HART, PROFIBUSPA, FOUNDATION Fieldbus and WirelessHART. All four of these protocols are designed to connect field instruments to control and monitoring host systems, typically a distributed control system (DCS) or an asset management system (Figure 1).

The main differences among the four protocols are the degree of functionality and corresponding complexity. Wired HART instruments typically only deliver one variable and have limited diagnostics, and implementation is relatively simple. PROFIBUS PA, FOUNDATION Fieldbus and WirelessHART instruments supply a number of variables and diagnostics messages, which can add to the implementation challenge. WirelessHART instruments also supply many variables and diagnostics, and do so with a degree of difficulty that falls closer to wired HART than FOUNDATION Fieldbus and PROFIBUS.

Each of the four protocols has its advantages, design considerations and maintenance challenges.

Wired HART was one of the first digital communication protocols and is the most widely used. The HART signal is superimposed on the 4-20mA wiring already in place to connect the field instrument to a host system or configuration tool (Figure 2). This makes it easy to install as no new wiring, power supplies, or conditioners are required.

Where a standard 4-20mA instrument exists, an equivalent HART instrument is almost always available, so upgrading the instrument is simple. Depending on which host system is used, some modifications may be required to accept and interpret the HART signal.

The 4-20mA signal is an industry standard and is proven in use to the point of acceptance in safety applications requiring SIS approvals. It is simple to add HART functionality to existing 4-20mA installations, and this is the least expensive option overall for adding functionality such as additional variables, ease of calibration and maintenance—particularly for retrofit applications.

The HART caveats are the slower transmission rates and the difficulty of getting additional variables out of the instrument. The digital data is limited to situations where real-time and/or deterministic data is not required.

The PROFIBUS PA protocol is related to the more familiar and widely used PROFIBUS DP standard (Figure 3). PROFIBUS DP is typically used in machine automation and other discrete signal applications. PROFIBUS PA is used for measurement and process control applications and can be used in hazardous areas.

PROFIBUS PA has the same physical layer as FOUNDATION Fieldbus, but it uses master/slave communications and thus has non-deterministic data transfer. Digital protocols such as PROFIBUS PA confer certain advantages, but also add design considerations. Different types of instruments use this protocol, and here are its main advantages:

FOUNDATION Fieldbus (FF) is the most widely deployed advanced fieldbus protocol, with millions of FF instruments installed worldwide. Its communications structure differs from PROFIBUS PA as it is peer-to-peer. This means instruments can communicate with each other, and can perform real-time control in the field as the data is deterministic. Its data structure differs from PROFIBUS as it allows custom blocks, and calculated or inferred variables.

FF was designed from the ground up as a high-speed, real-time instrument bus (Figure 4). Its strengths are:

WirelessHART is a self-organizing and self-healing network of instruments communicating wirelessly. The power of this network is combined with the simplicity of the HART protocol. Although mostly touted for its low installation cost, this protocol also features easy access to all process variables, health status, diagnostic information, calculated/inferred variables, and network parameters. This data makes it well suited to reliability, maintenance, quality and other applications (Figure 5).

As the Table shows, each protocol has its advantages and design considerations, but all share common requirements. Training for engineers implementing the project is essential as is the use of protocol planning tools, engineering guides, segment design recommendations, gateway loading and other design considerations.

Maintenance challenges can be effectively met by training of instrument technicians, purchasing the required tools, and planning for job descriptions that cross between instruments and host systems.

With these requirements satisfied, a blend of wired HART and WirelessHART is often best for projects where the objective is to simplify and speed calibration, instrument maintenance and troubleshooting at the lowest cost.

The HART standard has been in use for decades so many are familiar with installation, start-up, and maintenance of wired HART devices. These instruments can measure multiple process variables or infer variables, but getting these secondary variables out of the instrument can sometimes be a challenge.

PROFIBUS PA is often used in facilities with existing PROFIBUS DP installations, or on projects where the host system offers superior support for PROFIBUS protocols as compared to FF.

FF is the only wired fieldbus network designed from the ground up for field instrument and valve monitoring and control.

WirelessHART makes sense for process plants adding new points of measurement as it requires no signal wiring to instruments, and often no power wiring. Instruments can be added very quickly once the wireless infrastructure is in place, and wiring infrastructure and its required maintenance is much less than with other fieldbus types. In many cases, WirelessHART data transmission will be more reliable than wired systems because there are no wires which can corrode or be inadvertently damaged.

Wired has traditionally been the standard used for control, but wireless is maturing and being relied on more and more for real-time control, depending on application requirements. Going forward, new installations will use a seamless combination of both wired and wireless technologies. 

Medium for new installations, very low if wireless infrastructure is in place

Yes, except for very high-speed applications

Access to variables and diagnostic information

Dale Perry is the Wireless Service and Training Manager at Emerson Automation Solutions. He has over 32 years of service, training and product management experience in the process industries. Dale is a technical developer and instructor, and he leads worldwide seminars covering Emerson products and industry technologies.

Check out our free e-newsletters to read more great articles..

©2020 Automation.com, a subsidiary of ISA