Inside the Type 146 Unit
Chapter Three: Overview
68
Auxiliary Output Board - Fail-Safe Feature
Relays C and D on the Auxiliary Output board are associated with the 146 instrument’s Fail-Safe
feature. When the 146 unit is powered on, this feature is activated. The processor waits until
pressure readings are stable before activating any of the relays in the 146 unit.
The Fail-Safe feature includes a timed hardware reset that causes relays C and D to go to default
positions. The processor in the 146 unit prevents the timed hardware reset from executing by
sending out an interrupt pulse every 10 milliseconds. If the 146 unit’s software crashes, the
interrupt pulse stops and the timed hardware reset executes, causing relays C and D to go to
default positions.
Code 7x in the 146 diagnostics feature, is used to determine if the Fail-Safe system is operating,
and its response time. When Code 7x is used, the processor sends a relay setup command to
relays C and D, and then checks to see if the relays have responded. If the relays do not respond,
the abbreviation Err (for Error), appears in the main display. If the relays do respond, the
Fail-Safe test continues; the Code 7x function determines the Fail-Safe system response time,
and displays it in the main display (in milliseconds). The response time is the elapsed time
between when an interrupt pulse is received by the hardware reset, and when relays C and D go
to the default position (de-energized state).
To confirm that the Fail-Safe feature is operating properly, refer to Code 7x: Response Time for
Fail-Safe Feature, page 299.
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