Campbell Scientific 0871LH1 Freezing Rain Sensor - Manual (Page 20)

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Appendix A
1 RS-422 Output Format for non-Campbell Datalogger
Applications
This output operates at 9600 BAUD (One Start Bit, 8 Data Bits, No Parity,
One Stop Bit). A 24-byte string is sent once per second. See Section 9,
Table 3 for string definitions.
A two-line output provides a unidirectional serial port, running at 9600
BAUD (8-bits, one Start Bit, One Stop Bit, no parity), to allow
communication with aircraft electronics and external test equipment.
2 Built In Test (BIT)
Built-In-Test (BIT) capabilities of the freezing rain sensor consist of
hardware, continuous, power-up, and operator-initiated tests.
Whenever a failure is detected and verified, the freezing rain sensor stops
detecting and annunciating icing conditions and the heaters are
disabled. Failures detected in Initiated and Continuous BIT are counted
and enunciated once they have been verified. To eliminate nuisance
errors, failures are verified by delaying (debouncing) the failure for a
period of time. Failures detected in Initiated BIT are latched and power
must be cycled on and off to remove a failure. If failures detected in
Continuous BIT go away, the ice detector changes back to normal mode,
and once again enables all ice detection functions.
3 Hardware Built-In-Test (BIT)
Hardware BIT is comprised of a watchdog timer that forces the
microcontroller to re-initialize if it does not receive a strobe every 1.6
seconds. An internal voltage monitor forces the microcontroller to the
reset state if the internal 5VDC power supply falls below 4.65 VDC and
holds it there until the power supply returns above 4.65 VDC. When the
microcontroller is reset, no output string is sent.
4 Continuous Built-In-Test (BIT)
Continuous BIT consists of verifying the following:
• The probe heater is in the correct state. The return leg of the heater is
monitored.
• The ICE discrete output is in the correct state. The ICE discrete output
is fed back to the microcontroller through a passive voltage divider
and voltage comparator.