Process plant engineers of a certain age will remember the “fieldbus wars” from about 20 years ago. Digital fieldbus protocols were duking it out to see which one(s) was going to emerge triumphant as plants all moved from 4-20 mA analog instruments to digital. The war fizzled out when most companies stuck with their comfortable long-standing analog approach, and those that did change to a fieldbus effectively winnowed the list to two main choices.
In the years since then, there’s been a smaller skirmish with wireless instrumentation protocols, but the market has spoken and one choice has taken such a sizeable lead that it is evolving into a de facto standard. Even so, companies still have to decide what instrumentation communication protocol(s) they might want to use.
In January 2018, automation.com published an article by Emerson’s Dale Perry, How to Pick the Right Fieldbus Protocol, where he offers advice on choosing among four leading protocols, the first three wired and the last one wireless:
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.
Purists might note that these four aren’t all technically fieldbus platforms, but they do share the function of connecting field instruments to control and monitoring host systems. The question Dale tries to answer is why a given plant would choose one over another, of why it might use more than one depending on a specific application. Among the four, HART still reigns supreme. (Of course it has become a retronym and we now have to say wired HART to distinguish it from WirelessHART.)
Wired HART was one of the first digital communication protocols and is the most widely used. The HART signal is superimposed on the 4-20 mA wiring already in place to connect the field instrument to a host system or configuration tool. This makes it easy to install as no new wiring, power supplies, or conditioners are required.
The two “true” fieldbus platforms are more complicated:
The PROFIBUS PA protocol is related to the more familiar and widely used PROFIBUS DP standard. PROFIBUS PA is used for measurement and process control applications and can be used in hazardous areas. It 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.
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.
The newest of the group, although it’s now 10+ years old, is WirelessHART, and as the name implies, it’s the only one that does not depend on cables to carry data.
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.
The article is quite detailed and digs into a wide range of specifics to compare and contrast the various options. As Dale points out, this is not an all-or-nothing choice.
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.
You can find more information like this, and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the WirelessHART, instrumentation and other specialty groups for suggestions and answers.