14.5.3 Switch-closure and open-collector measurements
Switch-closure and open-collector (also called current-sinking) signals can be measured on
terminals:
l
P_SW or C
Mechanical switch-closures have a tendency to bounce before solidly closing. Unless filtered,
bounces can cause multiple counts per event. The data logger automatically filters bounce.
Because of the filtering, the maximum switch-closure frequency is less than the maximum high-
frequency measurement frequency. Sensors that commonly output a switch-closure or an open-
collector signal include:
l
Tipping-bucket rain gages
l
Switch-closure anemometers
l
Flow meters
Data output options include counts, frequency (Hz), and running average.
14.5.3.1 P_SW Terminal
An internal 100 kΩ pull-up resistor pulls an input to 3.3 VDC with the switch open, whereas a
switch-closure to ground pulls the input to 0 V.
l
CRBasic instruction:
PulseCount()
. See the CRBasic Editor help for detailed instruction
information and program examples:
https://help.campbellsci.com/crbasic/cr300/.
Switch Closure on P Terminal
Open Collector on P Terminal
14.5.3.2 C terminals
Switch-closure measurements on C terminals require a 100 k
Ω
pull-up resistor to 12 V. Switch-
closure mode is a special case edge-count function that measures dry-contact switch-closures or
open collectors. The operating system filters bounces.
14. Measurements
81