E-Zine September 2012
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Fieldbus creates a data set for each transaction consisting of the value of the desired variables/attributes and the status and mode of the device sending the message. In Fieldbus, status has a special meaning with respect to the quality of the data. The data may be classified as: good, uncertain, or bad. Data quality is determined by the measuring device based on its own diagnostics for the state of the sensor. If all is working as desired, the data is good and may be used for control. If there is a known problem in the sensor, such as being above or below the range established for that variable, the data is marked bad and may not be used for control. If the data is marked uncertain, the value of the process variable may be used for control if permitted by the control function block, but that there may be something wrong in the measurement system. For example, when two measurements of the same process variable are averaged to create the controlled variable, one of the two signals may be out-of-range and excluded from the average. The remaining signal is good and may be used for control, but the desired redundancy is lost. When a diagnostic indicates that a transmitter is outside its temperature or vibration limits, its measurement may also be marked dubious or uncertain.
Function block mode is related to the operating state of the function block and the communication of that state to other blocks. A control block must have a mode of either automatic or cascade to calculate its output using the process variable and setpoint values.
Excerpted from The Consumer Guide to Fieldbus Network Equipment for Process Control