Standards and Technologies for Harmonised Device Configuration

01 February 2007

Device profiles standardise device functionality as well as theirinput/output data. They play a key role for the open integration of devices into control systems and plant asset management systems.

The adjustment of a field device in order to adopt it to its particular purpose in a process automation plant, known as device configuration, is a crucial task during the engineering phase. Consider for example a radar tank level gauge. For this instrument, device configuration includes the definition of the measured value's meaning: will it be the distance from the field device to the liquid surface, the distance from the bottom of the vessel to the surface, or even the volume of liquid within the vessel? For measuring the surface height, the size of the vessel has to be known; for the volume calculation the shape and size of the vessel have to be taken into account. Will the volume be in litres or percent of tank, and so on.

In this article we will highlight different scenarios for device configuration. Remote configuration from a central tool, typically part of a Plant Asset Management System (PAMS), plays an important role especially for the managing of the device settings (storing, documentation, audit trail, etc.).

In order to be able to do offline device configuration--that is to say, the device is not physically available during the configuration process--technologies known as EDD and FDT/DTM technology have been established. For device configuration independent from the device manufacturer, a standard for at
least the core functionality of devices is required: Here profiles contribute considerably to open device integration into different plant asset management systems.

Device configuration can be done in different ways. For managing the device settings in a PAMS, remote access to the device parameters plays a key role, and, as we will see, the ability to do offline configuration is very important.

Local and remote configuration
The traditional and most simple way to perform device configuration is at the device itself. Usually the device is equipped with a display that indicates the measured value and, together with pushbuttons next to the display, the user can navigate through the device settings and modify device parameters.

Obviously, local device configuration requires that the device is available and powered. A disadvantage for large plants where devices have already been installed is that local device configuration might result in a time consuming and hence cost driving procedure, especially if the devices are widely spread in the plant and difficult to access. Local device configuration also requires manual documentation of the settings of each device.

For plant asset management systems, or PAMS, it is a great advantage to perform device configuration
remotely from a central station. To do this, it is necessary to have a communication technology for the
parameter data. In process automation three different technologies for digital communication have been
established. The most widespread is HART, which is based on a digital signal imposed on top of the
conventional 4...20 mA signal. Completely digital fieldbus systems such as Profibus PA and Foundation Fieldbus allow higher-speed transmission of process values as well as the transmission of parameterisation data.

Beside the remote access of field devices, the user benefits from other functions that come with the central management of device settings. Storing device settings electronically together with information about 'who did which change and when' and 'for what reason' creates an audit trail. The automatic processing of device settings for the purpose of documentation and archiving is another benefit. If it becomes necessary to replace one of the field devices, the configuration of the new device is straightforward: the operator simply downloads the stored settings into the new device.

'Plug-and-play' won't work
As an example, consider a USB device connected to a PC. After the USB connection has been established the device properties are retrieved so that the computer can operate it in the way it is designed, even if no device driver has been installed. However, a USB device can only be configured online; it has to be physically connected to the computer.

USB devices take only a few minutes to install and configure. In process automation, however, it sometimes takes much longer to install field instruments so, in order to save time, it is a strong requirement that installation and configuration occur in parallel. This is one of the reasons why remote configuration 'plug-and-play' has not been adopted.

In order to perform device configuration offline the PAMS requires some kind of 'proxy' of the device. (A proxy is something that functions as a substitute for another.) This proxy can be an electronic description or a software component that reflects the device's functionality. There needs to be a standard to define the device
functionality so that a device conforming to the standard--called a profile--can be configured offline within a PAMS supporting the standard.

The need for profiles
An example of a profile is the Profibus profile for PA devices (discussed below). Another profile is the Electronic Device Description (EDD), which has been standardised in IEC 61804-3. EDD describes device specific functionality in an ASCII file.

A somewhat different approach to profiles is the DTM, which is provided by FDT/DTM technology. This is an industry standard that specifies the communication between a so called FDT frame application (Field Device Tool) and a software component (Device Type Manager, DTM). A DTM can be opened from within the FDT frame application. It reflects the device functionality. The DTM is comparable to a device driver for a printer that is installed on a PC and that can be used from within a Windows application. In our example, the DTM is comparable to the device driver and the FDT frame application is comparable to the Windows operating system.

The device functionality that is standardised in a profile or described by an EDD or DTM concerns the
parameters of the device that are accessible such as parameter name, data type, address of the parameter, and if it is readable or writable.

A profile also standardises restrictions between parameters. An example of a restriction is a write locking parameter: if this parameter is set, all other parameters (even the writable ones) are read-only. Device functionality also concerns the processing of the process value (linearisation, limit check, simulation of process value) or device diagnostics.

A PAMS is usually based either on EDD or FDT/DTM technology which means that an EDD or DTM can be installed in the PAMS so that the particular device can be configured by the PAMS. Some PAMS vendors support both technologies. The integration of an EDD interpreter into a 'generic' DTM is an elegant way to combine both technologies. The profile specification is usually not implemented in the PAMS directly but in so called profile EDDs or profile DTMs. After a brief look into the Profibus profile for PA devices, profile EDDs and profile DTMs and their benefits will be discussed. Table 1 summarises the scenarios listed above
together with its online and offline configuration capabilities.

Profile for PA devices
The Profibus profile for PA devices (IEC 61784) specifies parameters and functionality of Profibus devices for Process Automation (PA). Referring to the ISO/OSI reference model of digital communication networks, a profile specifies the application layer (layer 7) of a device.

Part 1 of the profile defines general requirements for profile devices as the data types to be used and the device model. The device model consists of several blocks; there are three main types of blocks:

....A physical block (parameters that characterise the device as manufacturer ID, serial number or tag
description);

....Transducer blocks (parameters and functionality that are related to the measurement principle); and

....Function blocks (parameters and functionality to calculate the process value or to process the output value
from the control system, containing also the cyclic communications input/output data).

Part 2 of the profile defines by which communication services of the Profibus DP protocol the profile functionality is accessed (mapping of the profile to Profibus DP protocol). Also, the length and format of the input and output data for the cyclic data communication for each sort of device specified in parts 3 to 7 (below) are defined in part 2.

Parts 3 to 7 specify transducer and function blocks for several classes of PA devices as transmitter devices, discrete input and discrete output devices, actuators, and analysers.

The specification of a transducer block and a function block consists typically of a parameter description and a functional specification. The parameter description consists of the parameter name, an explanation of its meaning, and the data type of the parameter. The description specifies if it is a read-only parameter or if it also writable, if it is an input parameter (parameter value is required by the block functionality or processed
by the block), an output parameter (value is provided by the block, e.g. for another block) or a contained parameter (block internal parameter). The parameter description also specifies the relative address
(relative index) of the parameter so that a Profibus master is able to access the parameter value by acyclic read or write services. A block consists of four types of parameters: The standard parameters (mandatory) that are available in every block, mandatory profile parameters, optional profile parameters, and device specific parameters defined by the manufacturer (see figure 1).

The functionality of a block is specified within the parameter description and by means of state machines or signal flow diagrams.

Benefits for PA devices
The device functionality specified in the profile is reflected by a profile specific EDD (profile EDD) or a profile DTM. A profile EDD or a profile DTM can be used to configure a profile conforming device from any manufacturer in the scope of the PA profile. However, only the core functionality of the device can be configured. Manufacturer specific parameters are not available from within a profile specific EDD or a profile
specific DTM.

As mentioned above, ‘Part 2 of the profile’ specifies length and format of the input/output data during cyclic
communication. Based on this specification the so called profile GSDfiles (General Station Description) have been created that allow the set up of cyclic data communication with a Profibus master without requiring a manufacturer specific GSD file. (For downloading profile GSD files refer to www.profibus.com). The profile simplifies the integration of profile conforming devices into PAMS and control systems by profile EDDs, profile DTMs and profile GSDs and hence reduces the dependencies from manufacturers considerably.

Remote device configuration from a PAMS plays an important role for the managing of device settings in process automation. As we have seen, for offline device configuration a device description (EDD) or a device driver (DTM) fitting to the particular device is required.

The EDD approach as a text based device description, as well as the FDT/DTM approach based on software
components running in a FDT frame application have been established for device configuration. Profiles
standardise device functionality as well as their input/output data and allow the use of profile EDDs, profile DTMs and profile GSDs to configure the core functionality of a device and to set cyclic communication. Hence profiles play a key role for the open integration of devices into PAMS and into control systems.

Dr.-Ing. Matthias Römer, Endress+Hauser Process Solutions AG


Print this page | E-mail this page