FUNCTION BLOCKS

OVERVIEW
This section introduces fieldbus systems that are common to all
fieldbus devices.
INTRODUCTION
A fieldbus system is a distributed system composed of field devices and
control and monitoring equipment integrated into the physical
environment of a plant or factory. Fieldbus devices work together to
provide I/O and control for automated processes and operations. The
Fieldbus FOUNDATION provides a framework for describing these systems
as a collection of physical devices interconnected by a fieldbus network.
One of the ways the physical devices are used is to perform their
portion of the total system operation by implementing one or more
function blocks.
Function Blocks Function blocks within the field bus device perform the various
functions required for process control. Because each system is different,
the mix and configuration of functions are different. Therefore, the
Fieldbus FOUNDATION has designed a range of function blocks, each
addressing a different need.
Function blocks perform process control functions, such as analog input
(AI) and analog output (AO) functions as well as
proportional-integral-derivative (PID) functions.
The standard function blocks provide a common structure for defining function block inputs,
outputs, control parameters, events, alarms, and modes, and combining
them into a process that can be implemented within a single device or
over the fieldbus network. This simplifies the identification of
characteristics that are common to function blocks.
The Fieldbus FOUNDATION has established the function blocks by
defining a small set of parameters used in all function blocks called
universal parameters.
The FOUNDATION has also defined a standard set
of function block classes, such as input, output, control, and calculation
blocks. Each of these classes also has a small set of parameters
established for it. They have also published definitions for transducer
blocks commonly used with standard function blocks. Examples include
temperature, pressure, level, and flow transducer blocks.

The FOUNDATION specifications and definitions allow vendors to add
their own parameters by importing and subclassify specified classes.
This approach permits extending function block definitions as new
requirements are discovered and as technology advances.

. When execution begins, input parameter values from other blocks are
snapped-in by the block. The input snap process ensures that these
values do not change during the block execution. New values received
for these parameters do not affect the snapped values and will not be
used by the function block during the current execution.

Once the inputs are snapped, the algorithm operates on them,
generating outputs as it progresses. Algorithm executions are
controlled through the setting of contained parameters. Contained
parameters are internal to function blocks and do not appear as normal
input and output parameters. However, they may be accessed and
modified remotely, as specified by the function block.
Input events may affect the operation of the algorithm. An execution
control function regulates the receipt of input events and the
generation of output events during execution of the algorithm. Upon
completion of the algorithm, the data internal to the block is saved for
use in the next execution, and the output data is snapped, releasing it
for use by other function blocks.

A block is a tagged logical processing unit. The tag is the name of the
block. System management services locate a block by its tag. Thus the
service personnel need only know the tag of the block to access or
change the appropriate block parameters.
Function blocks are also capable of performing short-term data
collection and storage for reviewing their behavior.

Device Descriptions
Device Descriptions are specified tool definitions that are associated
with the function blocks. Device descriptions provide for the definition
and description of the function blocks and their parameters.
To promote consistency of definition and understanding, descriptive
information, such as data type and length, is maintained in the device
description. Device Descriptions are written using an open language
called the Device Description Language (DDL). Parameter transfers
between function blocks can be easily verified because all parameters
are described using the same language. Once written, the device
description can be stored on an external medium, such as a CD-ROM or
diskette. Users can then read the device description from the external
medium. The use of an open language in the device description permits

Introduction
interoperability of function blocks within devices from various vendors.
Additionally, human interface devices, such as operator consoles and
computers, do not have to be programmed specifically for each type of
device on the bus. Instead their displays and interactions with devices
are driven from the device descriptions.
Device descriptions may also include a set of processing routines called
methods. Methods provide a procedure for accessing and manipulating
parameters within a device.
BLOCK OPERATION
In addition to function blocks, fieldbus devices contain two other block
types to support the function blocks. These are the resource block and
the transducer block. The resource block contains the hardware specific
characteristics associated with a device. Transducer blocks couple the
function blocks to local input/output functions.

Function Blocks

Resource Blocks
Resource blocks contain the hardware specific characteristics
associated with a device; they have no input or output parameters. The
algorithm within a resource block monitors and controls the general
operation of the physical device hardware. The execution of this
algorithm is dependent on the characteristics of the physical device, as
defined by the manufacturer. As a result of this activity, the algorithm
may cause the generation of events. There is only one resource block
defined for a device. For example, when the mode of a resource block is
“out of service,” it impacts all of the other blocks.

Transducer Blocks
Transducer blocks connect function blocks to local input/output
functions. They read sensor hardware and write to effector (actuator)
hardware. This permits the transducer block to execute as frequently as
necessary to obtain good data from sensors and ensure proper writes to
the actuator without burdening the function blocks that use the data.
The transducer block also isolates the function block from the vendor
specific characteristics of the physical I/O.

Alerts
When an alert occurs, execution control sends an event notification and
waits a specified period of time for an acknowledgment to be received.
This occurs even if the condition that caused the alert no longer exists.
If the acknowledgment is not received within the pre-specified time-out
period, the event notification is retransmitted. This assures that alert
messages are not lost.
Two types of alerts are defined for the block, events and alarms. Events
are used to report a status change when a block leaves a particular
state, such as when a parameter crosses a threshold. Alarms not only
report a status change when a block leaves a particular state, but also
report when it returns back to that state.