Process Engineering: HART – Dealing with device description

There is a myth surrounding HART devices that you'll need a device description to communicate with a HART device. Learn why this isn't true and what you do need to know in operating a HART communication device.

By Ed T. Ladd, Jr.

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MYTH:  A Device Description (DD) must be used to communicate with a HART Device

FACT:  A DD is an optional but enabling element of the HART Communication technology and is not required to communicate with a HART device.

Over the past few years, there has been a great deal of furor over the use of the Device Description Language versus other technologies. Manufacturers have aligned themselves on either side of the issue and the HART Communication Foundation has stated, “The Device Description Language (DDL) is the only approved standard for electronically describing a HART-enabled device.” 

Ongoing efforts are currently reshaping current perceptions of the use of the Device Description Language. Enhancements to the language provide new and improved user interfaces as well as additions that complement the already available diagnostic capabilities of HART. These additions include the ability to embed images as well as the ability to display data on graphs and charts described in the Device Description (DD).

Even with the advent of these enhancements, discussions continue throughout the industry regarding DDL technology versus other technologies used to describe devices. Nevertheless, the fact remains:  a HART-enabled device does not require the user to have a Device Description.

Why?

The HART Communication Protocol operates using a host/slave relationship, where a device responds to a query from a host. A HART-compliant host queries a device using commands defined by the protocol. The HART commands provide for a base set of functionality allowing manufacturers a considerable amount of flexibility when designing a device.  

Every device that claims HART compatibility must adhere to this base set of command rules. All devices must implement the mandatory Universal Commands. Most devices also implement the optional Common Practice Commands. 

Universal and Common Practice Commands allow users to modify up to four dynamic variables, trim analog outputs, modify basic setup items such as tag or a descriptor, perform a reset, and access diagnostic information.

In addition to the Universal and Common Practice Commands, many devices implement manufacturer specific commands or Device Specific Commands. Device Description Language allows a manufacturer to create a Device Description detailing these commands and allowing access to all information available in the device whether Universal, Common Practice or Device Specific. 

What You Need To Know

Users’ needs for DD are dictated by the type of host system that is being implemented. Typically, control or automation systems have a different need compared to an asset management system. 

Using HART, automation systems typically do not require the use of a DD to learn a devices status. These types of hosts get information from the device using Universal, Common Practice Commands and the analog loop. Automation systems will use the analog loop to determine the value of the primary control variable and Universal / Common Practice Commands to verify or validate loop integrity, obtain device status and capture additional process related variables.

Technology improvements have made intelligent devices increasingly more powerful, adding functionality and device specific diagnostics that can be interpreted through using a DD. Asset / Maintenance Management systems use the DD to fully understand the functionality built into a device. By accessing these features modeled by the DD, systems enable plant personnel to modify device configurations and understand potential problems with a device.      

Device Description-enabled host systems are designed to use all the data present in a device, displaying it, as the user requires. When evaluating any HART host, users should first determine their requirements. Are you dealing with an Asset Management or a Control/Automation System? Are your devices complex (e.g. Mass Coriolis) or simple (e.g. temperature transmitter)?  Will you need to access Device Specific Commands to service your device?  Knowing the answers to a few simple questions can move you in the right direction.

When evaluating a host, users should ask the following questions:
• Does your host support all Universal and Common Practice Commands?
• Is your host Device Description enabled?
• Does your host use the HCF’s standard binary file format?
• Is there an easy way to upgrade DDs from the HCF DD Library?

Additional questions to ask your device vendor:
• Are your DDs registered with the HCF DD Library?
• Are your devices registered with the Foundation?

If the answer to these questions is yes, at a minimum you should be able to use all of your HART devices with the host you select.  By doing your due diligence up front you confirm there will be no information gaps when using HART-enabled devices.

Close the information gap and “See What You Can Do” when you use the “Power of HART”.

Upcoming HART Connection articles will provide information on applying HART technology, application notes to help you implement new HART strategies and what to expect from a HART-enabled device or host system. For more information, contact the HART Communication Foundation or visit the website at http://www.hartcomm.org.

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