Fieldbus - Computer Busses - page 309

• Manufacturer ID.
• Device type of device.
• Revision.
• Memory usage and free space.
• Computational time.
• Device state (on-line/off-line/standby/fault condition/etc).
19.3.5 Transducer block (TB)
Each device has a TB, which the named entity that stores the parameters associated with the
sensor or actuator.
19.3.6 Alarm notifications
Each block (functional, resource or transducer) can produce an alarm notification which is
associated with that particular block, such as:
• Process problems with function blocks.
• Sensor/actuator problems with transducer blocks.
• Overall device problems with resource blocks.
The devices each have parameters, which are structured using an object dictionary, which is
a standardised method of interrogating and referencing parameters over the communications
link.
In FOUNDATION Fieldbus the trip-point values are low, low-low (LO_LO_LIM), high
(HI_LIM), high-high (HI_HI_LIM), deviation-low (DEV_LO) and deviation-high
(DEV_HI). These trip values can generate alarms, which have certain priority levels. These
are:
• Priority 0 - disables alarms including the setting of the alarm condition status flag.
• Priority 1 - disable report, but causes the status flag to be set.
• Priority 2 to 7 - advisory alarms.
• Priority 8 to 15 - critical alarms.
19.3.7 Device description language (DDL)
The FBs, TBs and RBs are not just limited to a standardized set of parameters, a new DDL
allows manufactures to specify additional parameters in a standardised manner. This includes
names, data types, enumerations, units, valid ranges, user entry limits, entry conditions (such
as out-of service or manual mode), connection properties, presentation information and help
text. Updates can be easily installed with DDs (device descriptions), which is a compiled
form of DDL. This allows easy updates and bug fixes on equipment, as updates can be
downloaded onto the equipment.
19.3.8 Control
The two main control advantages of the Fieldbus are that it truly distributes control and that
control processing can be done concurrently (rather than being centralised in a controller).
The devices can implement many of the control functions that a traditional DCS would do.
Most of the control functions are implemented by the following: