Sensoray 518 - Manual (Page 51)

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Theory of Operation
Software
Execution of the internal coprocessor software is managed by a resident multi-tasking kernel.
The kernel is responsible for scheduling and executing concurrent foreground tasks, processing
intertask communication requests, and servicing real-time communication interrupts from the
system bus interface.
The following paragraphs describe the inner-workings of tasks that are controlled by the kernel.
Scanner
This task performs the fundamental data acquisition function central to the coprocessor’s
purpose.
The scanner begins a data conversion by selecting the next channel to be digitized. The analog
front-end multiplexers are switched to the desired channel, and the pulsed excitation source is
activated if the channel is connected to a resistive sensor type. The scanner then goes to sleep
while the sensor signal settles out.
When the sensor signal stabilizes, the scanner initiates an A/D conversion. Other tasks are
allowed to run while the conversion is taking place.
Upon completion of the conversion, the A/D data is sent to the post-processor task.
Post-processor
The post-processor is responsible for refining uncorrected A/D data into finished form.
This task begins by receiving uncorrected channel data from the scanner task. The data is
corrected for circuit offset and gain errors, then linearized and converted to engineering units
appropriate for the declared sensor type. The resulting value is passed through a single-pole low-
pass filter.
Next, the filtered data is checked for alarm limit violations. If a limit violation is detected , the
violated channel’s limits are both reset to their default values, the channel high or low alarm
status flag is asserted, and the status register alarm bit is set.
Finally, the processed channel data is stored in RAM for host processor access.