Safety Features And Fault Detection - CADD Legacy Plus 6500 Operator's Manual

Section 5: Reference

Safety Features and Fault Detection

Hardware Safety Features
Key hardware safety features include a watchdog timer circuit, motor
driver and motor watchdog circuits, and a voltage detector circuit. Each
safety circuit performs a unique function to insure the overall safety of
the device.
Watchdog Timer Circuit
The microprocessor must send an appropriate signal to the watchdog cir-
cuit at least once per second. If the microprocessor does not, the watch-
dog circuit will time out and shut down the pump controller.
Watchdog timer circuitry is provided to monitor the status of the mi-
croprocessor and disable the motor and enable the audible alarm if the
microprocessor fails to function properly. The microprocessor must
strobe the watchdog circuit at least once every second in order to pre-
vent the watchdog from performing its reset function. The reset output
from the watchdog circuit is a pulse output. This acts to "jump start" the
microprocessor. This unique feature allows the microprocessor to test the
watchdog circuit on every power-up.
By setting a flag in the memory and not strobing the watchdog, the
microprocessor can force a watchdog time-out. After being reset, the
microprocessor checks the status flag to see if this was a time-out test. If
so, the microprocessor continues normal power-up activities. If the reset
occurred when the microprocessor was not expecting it, the micropro-
cessor traps the event, sounds the audible alarm and displays an error
message on the LCD.
Motor Driver/Motor Watchdog Circuit
Motor drive circuitry is composed of a series of power FET transistors,
passive components, and 2 voltage comparators. Built into the motor
drive circuitry is an RC timer which times how long the motor runs
each time it is turned on. If the motor runs for more than an average
of 3 seconds, the circuit will time out and disable the motor. A unique
feature of this circuit is that control lines to and from the microprocessor
circuit allow the microprocessor to perform a complete functional test of