Emerson Rosemount OCX 8800, Инструкция по эксплуатации

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Section 7: Foundation
Fieldbus
April 2017
Instruction Manual
IM-106-880C, OI
92
Instrument-Specific Function
Blocks
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.
7.2.5 Instrument-Specific Function Blocks
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.
7.3 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.
7.4 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.
7.4.1 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 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.