Revision 3.1 - 56 - March 2010
The device vendor shall mark the diagnostic TB parameter as such in the DD (Device
Description Files) for ease of access. Furthermore, a clear Help File shall be added to each
parameter clarifying the device vendor’s knowledge on how to use the parameter information.
The Fieldbus Foundation has defined several standard TBs including pressure, temperature,
valves and some others. Devices that conform to these specifications are preferred.
5.4.3 Function Blocks
Function Blocks (FBs) should be employed in user-defined Function Block applications to
provide various functions required in a control system (for example, input, output, signal
selection, and other control actions). In other words, Function Blocks are the control strategy.
FBs are built into FOUNDATION devices, as needed, to achieve the desired control functionality.
Some FBs can be built into the devices and some can be instantiated (preferably at the
factory). The factory instantiation option should be used as it results in less confusion and
configuration effort. Reference should be made to the manufacturer's device manual for
optimal configuration.
Depending on the DCS (Host) manufacturer's policy, some of the specific device parameters
may not be shown. In the case of diagnostics this may reduce the device functionality.
Reference should be made to the manufacturer's device manual for optimal configuration. Not
all available FB types may yet be included in the present ITK tick marking. However, not all
FBs are required. Some FBs are only of interest if Control in the Field (CIF) is applied. Then, it
is essential to make a considered choice when specifying the Function Blocks to be included in
various field device types.
Commentary:
It is recommended to employ commonly used Function Blocks available in both the Host and
field device. Use specialty Function Blocks with great care.
Although appropriate to Host system controllers, these blocks may be limited to the following:
• AI for transmitters
• AO and PID for valves
• DI/DO for discrete devices
Additional Function Blocks are likely to be added in the future. It is, therefore, wise to check
Function Block availability with the instrument manufacturer at the time of purchase, thus
ensuring that the features desired are available for use. However, the interoperability of other
than the essential FBs is not guaranteed with every Host system, thereby limiting the CIF
applicability.
The Fieldbus Foundation tests of Function Blocks only confirm that they are present and how
their external interface behaves, not how well they work internally. Each manufacturer can
configure the internal operations of Function Blocks as they wish and will, in fact, do so since
this will give them a competitive advantage. It is thus worthwhile to check which manufacturer