🍵 HART Concept

The HART Protocol standardizes many data in field devices. Standardized access to process values, basic calibration, device and process status, and device identifying data are all supplied. These data are accessed via Universal, Common Practice and Device Family Commands. However, device specific data and properties are normally present as well.

Many of the value added capabilities and the detailed unique configuration properties of the device are found in the device specific commands. Since field devices are getting more and more complex this data can be quite extensive. Increasingly access to the device’s capabilities are being provided via HART. This is replacing or making optional local user interface panels which, in turn, reduces the cost of HART compatible field devices. This growing reliance and utilization of HART requires host applications that can review and configure device properties and behaviors.

In fact, users demand that the application they choose has complete and universal access to all device specific data. They demand their host application supports every HART compatible device in their plant. Different applications for different devices is simply too burdensome and unacceptable.

Standardization on HART is intended to reduce the number of tools that must be supported and the associated training that requires. With large number of HART compatible field devices these end user expectations place great demands on host applications. Furthermore, host developers must provide support for new field devices at an ever increasing rate. The concept of a device description language arose from discussion held in about 1990 for fieldbus.This idea was first made a reality with the development of the HART Device Description Language. This led to DDL for FF and PROFIBUS. DDL is a text based language for modeling devices that is independent of host platform, operating system and technology.

DDL models the real-time database in devices, communication with the device, and the standard operating procedures used by the device. Most software developers will find DDL very different from other languages they may encounter. DDL is declarative (not procedural) and does not tell the host how to does its job. DDL tells the host application what the capabilities of the field device are and it is up to the application to determine how to utilize the capabilities to meet its mission.

DDL can describe very complex device behavior. For example, in certain device operating modes data is available can be modified and in other modes they may not exist at all. Modifying one datum may affect the values of many data. DDL allows the device developer to describe these complex functionalities.With DDL, the device developer can organize the device data, properties and procedures for access by the end user. This provides the host application guidance in dynamically creating a user interface for the device. This user interface can vary significantly from device to device. The data, property and procedure grouping will be as simple or complex as the device DDL describes. DD's are available world-wide from the HCF's DD Library. New or upgraded DD's can be loaded into existing host applications providing support for devices that may not have existed when that application was originally sold. This provides the device developers great flexibility in introducing their products into the field. New devices can be released independently of host applications. Once a DD for the device exists, it can be loaded into the host application and will then be able to operate with it. Device developers are no longer required to verify the host application; they will only need to verify their DD. New devices, hosts, and new versions of existing devices and hosts can all be released independently and without impacting each other.

DDL does not replace the device’s technical documentation (see HCF_LIT-18) This document provides performance details and other technical information beyond the application layer data, properties and procedure found in a DD. DDL does enable universal host applications. DDL brings together in one place all the information a host application needs to operate with field devices. These applications can access all data and properties of all devices simplifying maintenance, support and operation of HART compliant devices. It works well with small handheld hosts and large integrated maintenance systems. It works with embedded applications and applications running on commercial operating systems. DDL's power and portability saves costs for host and device vendors alike.