Virtual Field Devices

A fieldbus device may have user applications, which are independent from each other and do not interact. A fieldbus device consists of Virtual Field Devices (VFDs) for such individual applications. VFDs can be seen as different field devices from an application point of view. Communication services guarantee their independence. Figure A-16 for relationship between VFD and communication.

VFDs in a Fieldbus Device: A FOUNDATION fieldbus device has at least two VFDs. One is the Management VFD where network and system management applications reside. It is used to configure network parameters including VCRs as well as to manage devices on a fieldbus. The other is a Function Block VFD where Function Blocks exist. It is possible for a field device to have two or more Function Block VFDs.

Function Block

Here we focuses on the most important concept of Function Block in FOUNDATION fieldbus, especially its models and parameters, through which you can configure, maintain and customize your applications.

What is a Function Block?

A Function Block is a functional model common in measurement and control. It is a generalized concept of the functionality you have in field instruments and control system such as analog input and output as well as PID control.

Function Block parameters are visible and accessible through communication services and the Block behavior depends on the values of parameters. A Function Block may reside in virtually any device on the network and a set of Function Blocks connected to each other to form an application can reside in one device or be distributed among devices. Fieldbus Foundation’s System Architecture document says:

“One of these models, the function block model,has been specified within the architecture to
support low level functions found in manufacturing and process control. Function
Blocks model elementary field device functions, such as analog input (AI) functions and
proportional integral derivative (PID) functions.
The function block model has been supplemented by the transducer block model to
decouple function blocks from sensor and actuator specifics. Additional models, such as
the ‘exchange block’ model, are defined for remote input/output and programmable devices.
The function block model provides a common structure for defining function block inputs,
outputs, algorithms and control parameters and combining them into an Application Process that
can be implemented within a single device. This structure simplifies the identification and
standardization of characteristics that are common to function blocks.”

The Function Block VFD contains three classes of blocks: Resource Block, Function Block and Transducer Block.

Resource Block A Resource Block shows what is in the VFD. It gives you the manufacturer’s name, device name, DD and so on. If the VFD allows you to create or download a Function Block, Resource Block shows how much resource (memory and CPU time) is available. Status of hardware is also visible. Resource Block controls the overall device hardware and Function Blocks within the VFD.

Function Blocks A Function Block is a generalized model of measurement and control. For example, the AI block conditions raw data from transducer(s) and provides the measured value in a common format. Function Blocks are classified into three classes: (1) a Standard Block as specified by the Fieldbus Foundation, (2) an Enhanced Block with additional parameters and algorithm, and (3) an Open Block or a Vendor-specific Block designed by individual vendors. A Function Block has input, output and contained parameters. Data generated in a block is exposed in an output parameter, which can be linked to the input parameter of other Function Blocks.

Transducer Blocks A Transducer Block is a model of sensors and actuators. It is modeled to give a similar expression to Function Blocks. Traditional sensors like pressure transmitters can be mapped to a Transducer Block. A Transducer Block is linked to a Function Block through the CHANNEL parameter of the Function Block. A Function Block is a general idea while the Transducer Block is dependent on its hardware and principles of measurement. For example, pressure transmitter and magnetic flowmeter have different measurement principles but provide an analog measured value. The common part is modeled as an Analog Input (AI) block. The difference is modeled as Transducer Blocks that give you the information on the measurement principle.