With the advent of Fieldbus devices, smarter instrumentation and sensors it is now possible to create large complex systems where control functionality is physically distributed across many devices. For example, think of an intelligent pressure sensor on a communications bus that is linked across a plant to a PLC that in turn drives a pump actuator, all monitored from a display panel. The control scheme is encompassed in the way the software in these different devices is connected.

BOTH data and events flow between IEC 61449 function blocks. Note that the function blocks are divided into top and bottom parts, with the top explicitly dedicated to event flow - an IEC 61499 innovation. Events are actions that trigger something to happen, such as an operator pressing a pushbutton, or a threshold value being reached.

Up to now there has been no consistent way to design and model how the software in such devices is interconnected across communications networks. The new IEC 61499 standard addresses this problem by providing a means to describe software that runs in industrial devices in terms of function blocks. IEC 61499 can be used to show how both events and data are passed between function blocks running in different devices, and how the function blocks respond to events and run internal algorithms.

For example, a smart pressure sensor could be defined as an embedded analogue input function block that provides a defined set of inputs for parameterisation and outputs such as the measured pressure value, sensor calibration status and error status. With IEC 61499, these inputs and outputs can be linked to inputs and outputs on any other function block in any device anywhere in a distributed system. For example the pressure sensor error status could be soft-wired directly to a function block driving an alarm display. This approach gives rise to an advanced level of flexibility and the possibility of improved and novel forms of control system design.

The IEC 61499 standard comes from a growing interest in new technologies and architectures for creating the next generation of distributed systems for industrial automation. These will be systems where software is organised as sets of co-operating components rather than as the integration of large custom-built units of software. In such systems, each device connected to the industrial network can provide part of the control functionality.

The industrial community has long been aware that the ready interconnection of software components, such as in the form of function blocks, will have major advantages especially for end-users. These advantages will include improved software productivity through re-use of standard solutions and improved design flexibility by being able to use plug-and-play software and devices from different vendors.

This standard, developed under the direction of the IEC, is based on function blocks - an established concept for defining robust, re-usable software components. A function block can provide a software solution to a small problem, such as the control of a valve, or control a major unit of plant, such as a complete production line. Function blocks allow industrial algorithms to be encapsulated in a form that can be readily understood and applied by people who are not software specialists.

The IEC working group for IEC 61499 has members from USA, Japan, UK and many European countries who represent both industrial control system suppliers and end-users. They have created this new standard based on function block concepts defined in the PLC language standard IEC 61131-3 in liaison with related concepts coming from the Fieldbus standardisation work.

The standard defines a general model and methodology for describing function blocks in a format that is independent of implementation and can be used by system designers to construct distributed control systems.

In this IEC 61499 function block diagram of a temperature control system, the block at the top, FB_Scan1 generates timing events. Note that its signal lines, which transmit event signal, emanate from the top portion of th function block and are propagated through the top portions of all the other function blocks in the system. Below it is FT_Temp1, a service interface function block that reads the process temperature and passes the values on to the PID function block, FB_PID1 which calculates an output value. The setpoint and PID parameters are input to this block from external sources. FB_HEAT1 is another service interface function block that writes out new output values to the heater.

As the trend to use component based software continues, it is foreseen that industrial controllers and instruments will either provide function blocks as part of the device firmware or provide function block libraries from which blocks can be selected and downloaded. System design will become the process of software component selection, configuration and interconnection, just as much as electronic hardware design is now primarily concerned with the selection and interconnection of IC chips.

IEC 61499 is a new PAS (Public Accessible Standard) and is only just starting to gain interest from the industrial community. But it is foreseen that control system design tools and products will start to emerge as the benefits of this new approach to system design are appreciated. Rockwell has shown interest; Siemens has recently introduced a new system called 'Profinet/CBA' (component based automation) which is planned to have an IEC 61499 compliant mapping in a later release.

The Industrial Automation Open Networking Alliance (IAONA) organisation has also published papers mentioning interest in using the standard particularly related to Fieldbus devices.

IEC 61499 is being developed as a multi-part standard. Part 1, which covers the basic constructs and architecture, was published at the end of 2000 and part 2 that covers support tool requirements was published in summer of 2001. Parts 3 and 4 are currently under development by the IEC working group WG6. The latter parts will cover topics such as compliance to the standard, programming tools to interface with a network, etc.