Campbell Scientific CR1000 Measurement and Control System - Manual (Page 316)

Section 8. Operation
316
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).
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 <1.5 Vdc to >3.5 Vdc) or falling edges (transitions
from >3.5 Vdc to <1.5 Vdc) of a square-wave signal can be counted.
Switch Closure (C1 - C8)
Two schemes are available for connecting switch-closure sensors to the CR1000.
If a switch is to close directly to ground, an external pull-up resistor is should be
used as shown in figure Using a Pull-up Resistor on Digital I/O Channels C1 - C8
(p. 318).
Alternatively, if the switch is to close ground through a digital I/O port,
connect the sensor to the CR1000 as diagrammed in figure Connecting Switch
Closures to Digital I/O
(p. 317).
Mechanical switch closures have a tendency to bounce before solidly closing.
Bouncing can cause multiple counts. The CR1000 incorporates software switch
debounce in switch-closure mode for channels C1 – C8.
Note Maximum switch-closure measurement frequency of C1 – C8 is 150 kHz.
8.1.5.2.2 Low-Frequency Mode
Low-frequency mode enables edge timing and measurement of period (not period
averaging) and frequency. For information on period averaging, see Period
Averaging
(p. 322).
Edge Timing (C1 - C8)
Time between pulse edges can be measured. Results can be expressed in terms of
microseconds or Hertz. To read more concerning edge timing, refer to CRBasic
Editor Help for the TimerIO() instruction. Edge-timing resolution is
approximately .
Edge Timing (C1 - C8)
Open collector (bipolar transistors) or open drain (MOSFET) sensors are typically
measured as frequency sensors. Channels C1 – C8 can be conditioned for open
collector or open drain with an external pull-up resistor as shown in figure Using
a Pull-up Resistor on Digital I/O Channels C1 - C8
(p. 318).
The pull-up resistor
counteracts an internal 100-kΩ pull-down resistor, allowing inputs to be pulled to
> 3.8 V for reliable measurements.