CR1000 Manual - Campbell Scientific

Section 8. Operation
8.1.5.2.1 High Frequency Mode
Digital I/O channels have a small 25-ns input RC-filter time constant between the
terminal block and the CMOS input buffer, which allows for higher-frequency
pulse counting (up to 400 kHz) when compared with pulse-input channels P1 –
P2 (250 kHz maximum).
8.1.5.2.2 Low-Frequency Mode
Switch-closure mode is a special case edge-count function. Because of signal
conditioning for debounce, 150 Hz is the maximum input frequency at which
switch closures can be measured on digital I/O channels.
Edge Counting (C1 - C8)
Rising edges (transitions from 3.5 Vdc) or falling edges (transitions
from >3.5 Vdc to 3.8 V for reliable measurements.
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