16
Cassette ‘Type’ Sensor Circuit
The cassette ‘Type’ sensor system consists of
three pins protruding from the button of the
pump mechanism that interface to the attached
administration set and associated circuitry.
Each type of administration set designed to
work with the CADD‑Prizm
®
PCS II pump
contains a unique ‘code’ programmed into the
set via nubs molded into the plastic. When a set
is latched to the pump, the nubs press against
the pins in the pump mechanism in a pattern
unique to that set type. Optical detectors and
electronic circuitry on the circuit board encode
this pattern and report the information to the
microprocessor. This feature allows automatic
rate selection dependent on the type of set
attached. This system also acts as a safety
feature to detect a damaged or detached set.
If, during operation, the microprocessor
detects all pins extended, the pump will enable
audible and visual alarms and stop delivery.
Redundancy in the pattern prevents single fault
failures from causing over or under delivery of
fluid. Additional circuitry allows these sensors
to be turned on and off by the microprocessor
to conserve battery power. Additionally, control
of sensor power allows the microprocessor to
test the sensor inputs in both the powered and
unpowered states, thus allowing detection of
sensor fault conditions. Care should be taken
not to damage these sensor pins.
Latch/Lock Sensor Circuit
Latch and Lock sensors allow the
microprocessor to detect the positions of
the latch and lock buttons. This prevents
attempted fluid delivery when the set is not
correctly latched to the pump. In addition,
it allows the microprocessor to stop fluid
delivery and enable audible and visual alarms
if the set is unlatched during fluid delivery.
Opposing infrared transmitters and receivers
on both the latch and lock buttons allow the
microprocessor to detect their open and
closed positions. Additional circuitry allows
these sensors to be turned on and off by the
microprocessor to conserve battery power.
Additionally, control of sensor power allows
the microprocessor to test the sensor inputs in
both the powered and unpowered states, thus
allowing detection of sensor fault conditions.
Voltage Detector Circuit
Low voltage detection is performed by
part of the watchdog circuit and by the
microprocessor via software. Three low
voltage levels are detected. The first two
levels are detected by software and the third
by hardware. The first level to be reached is
the Low Battery Warning threshold which
occurs when the battery voltage decays
to a nominal value of 6.8 volts. An Analog
to Digital Converter (ADC) built into the
microprocessor allows the microprocessor,
via software, to monitor the battery voltage.
At the Low Battery Warning threshold, the
microprocessor enables a periodic series
of beeps and displays a low battery warning
message on the LCD. As the battery voltage
reaches a nominal value of 6.3 volts, the
software disables delivery, places a battery
depleted message on the LCD, and enables a
constant two‑tone audible alarm. When the
battery voltage decays to a nominal value of
5.6 volts, a hardware reset circuit is triggered
which places the microprocessor in reset. This
prevents ambiguous microprocessor operation
when the battery voltage continues to decay.
The hardware reset continues until the battery
is completely discharged or until it is removed.
Once the pump controller goes into low battery
shutdown, only replacing the old battery with a
fresh one will clear the condition.
