Specifications

Section 4. Sensor Support
4-32
When a pulse channel is configured for high-frequency pulse, an internal 100
kΩ pull-up resistor to +5 V on the P1 or P2 input is employed. This pull-up
resistor accommodates open-collector (open-drain) output devices for high-
frequency input.
4.5.1.2 Low-Level AC
Rotating magnetic pickup sensors commonly generate AC output voltages
ranging from millivolts at low rotational speeds to several volts at high
rotational speeds. Channels P1 and P2 contain internal signal conditioning
hardware for measuring low-level AC output sensors. When configured for
low-level AC, P1 and P2 measure signals ranging from 20 mV RMS (±28 mV
peak) to 14 V RMS (±20 V peak). Internal AC coupling is incorporated in the
low-level AC hardware to eliminate DC offset voltages of up to ±0.5 V.
4.5.1.3 Switch Closure
Switch-closure mode of channels P1 and P2 measures switch closure events,
such as occur with a common tipping bucket rain gage. An internal 100 kΩ
pull-up resistor pulls the P1 or P2 input to +5 V with the switch open, whereas
a switch closure to ground pulls the P1 or P2 input voltage to 0 V. An internal
3.3 ms time constant RC debounce filter is used to eliminate multiple counts
from a single switch closure event.
4.5.2 Digital I/O Ports for Pulse Counting
Read more! Review digital I/O port specifications in Section 3.3. Review
pulse counter programming in CRBASIC Help for the PulseCount()
instruction.
Digital I/O Ports C1 – C8 can be configured for high-frequency input or switch
closure counting. Low-level AC mode is not available.
A 4 channel low-level AC signal conditioning peripheral
available from Campbell Scientific (LLAC4 ) generates up to 4
signal conditioned digital outputs that can be measured with
digital I/O ports.
Ports C1 – C8 have a small 25 ns input RC filter time constant between the
terminal block and the CMOS input buffer, allowing for higher frequency
operation (400 kHz maximum) as compared with the high-frequency pulse
mode of channels P1 and P2 (250 kHz maximum).
When configured for input, ports C1 – C8 each go into a digital CMOS input
buffer that recognizes an input voltage ≥ 3.8 V as a high (one) level, and an
input voltage ≤ 1.2 V as a low (zero) level. Voltage levels < -8.0 V and > 16 V
should not be connected to ports C1 – C8.
When using ports C1 – C8 for switch closure, an external pull-up resistor is
required to counteract the internal 100 kΩ pull-down resistor to ground. The
external pull-up must pull the input to > 3.8 V with the switch open for reliable
switch closure measurements. A pull-up resistor of ≤ 20 kΩ is recommended
when connecting to a +5 V supply, and a pull-up resistor of ≤ 150 kΩ is
recommended when connecting to a +12 V supply to provide adequate logic
NOTE