Rosemount OCX 8800, Instruction Manual

Page: 104 / 266

Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-4
Instrument-Specific
Function Blocks
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.
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.
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 acknowledgment
is not received within the prespecified 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.
Network Communication
Figure 7-2 illustrates a simple Fieldbus network consisting of a single
segment (link).