Chapter 5 _________________________________________________________ Theory of Operation
VAISALA _______________________________________________________________________ 167
Only a failure in the surface temperature measurement will generate an
alarm. This is the TEMPERATURE SENSOR FAILURE alarm, which is
generated when the measurement is not in the valid range. The other
temperature measurements are only for monitoring purposes and have no
effect on the operation of the unit.
Memory Tests and Program Operation
After a reset, the FS11P sensor clears and initializes its SRAM data
memory.
The checksum of the FLASH parameter memory is calculated and
checked with a test after the reset. An error in the checksum causes a
memory error report.
The program operation is monitored by the watchdog circuitry. If the
circuit is not triggered every second, it will perform a hardware reset.
Normal operation of FSC102 and FSC202 boards is indicated by green
status LEDs that blink once a second. The status LED of FSC202 is
located on the cover of FSC202 inside the interface unit FSI102. The
status LED of FSC102 is located inside the round center cover of
FSM102, which has to be removed to gain access to the LED. If the
green status LED is flashing once, there is an indication status on the
board. If the green status LED is flashing twice, there is a warning status
on the board. If the green status LED is flashing three times, there is an
alarm status on the board and finally, if the green status LED is flashing
four times, there is an error status on the board.
Communication Monitoring
The validity of the internal communication between Communication
Controller FSC202, Measurement Unit FSM102, and Background
Luminance Sensor LM21 and Precipitation Identifier PWD32 are
verified by the CRC16 checksum that is used in all internal
communication. For maintenance purposes the amount of other
communication errors is stored in an error log and can be read with the
advanced level command ELOG.
All data values have individual validity times. The typical timeout time is
1.5 times the measurement interval of the parameter, depending on the
parameter. If the data values are not updated within the timeout time and
the data is critical for visibility measurement, the data value is set as
missing (/////). The reason for the missing data value is shown in the
status message.
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