Campbell Scientific CR1000 Measurement and Control System - Manual (Page 315)

Section 8. Operation
315
8.1.5.1.1 High-frequency Pulse (P1 - P2)
High-frequency pulse inputs are routed to an inverting CMOS input buffer with
input hysteresis. The CMOS input buffer is an output zero level with its input
≥
2.2 V, and an output one level with its input ≤ 0.9 V. When a pulse channel is
configured for high-frequency pulse, an internal 100-k Ω pull-up resistor to 5 Vdc
on the P1 or P2 input is automatically employed. This pull-up resistor
accommodates open-collector (open-drain) output devices for high-frequency
input.
8.1.5.1.2 Low-Level ac (P1 - P2)
Rotating magnetic-pickup sensors commonly generate ac output voltages ranging
from thousandths of Volts at low rotational speeds to several volts at high
rotational speeds. Pulse channels contain internal signal-conditioning hardware
for measuring low-level ac-output sensors. When configured for low-level ac, P1
through 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 Vdc.
8.1.5.1.3 Switch Closure (P1 - P2)
Switch-closure mode measures switch closure events, such as occur with a
common tipping bucket rain gage. An internal 100-k
Ω pull-up resistor pulls an
input to 5 Vdc with the switch open, whereas a switch closure to ground pulls the
input to 0 V. An internal 3.3-ms time-constant RC-debounce filter eliminates
multiple counts from a single switch closure event.
8.1.5.2 Pulse Input on Digital I/O Channels C1 - C8
Digital I/O channels C1 – C8 can be used to measure pulse inputs between -8.0
and +16 Vdc. Low frequency mode (<1 kHz) allows for edge timing and
measurement of period and frequency. High-frequency mode (up to 400 kHz)
allows for edge counting only. Switch-closure mode enables measurement of dry-
contact switch closures up to 150 Hz. Digital I/O channels can be programmed
with either PulseCount() or TimerIO() instructions.
When configured for input, signals connected to C1 – C8 are each directed into a
digital-CMOS input buffer that recognizes inputs ≥ 3.8 V as being high and inputs
≤ 1.2 V as being low.
Low-level ac signals cannot be measured directly by digital I/O channels. Refer
to the appendix Pulse / Frequency Input-Expansion Modules
(p. 560)
for information
on peripheral modules available to convert low-level ac signals to square-wave
signals.
Read More! Review digital I/O channel specifications in CR1000 Specifications
.
Caution Contact Campbell Scientific for signal conditioning information if a
pulse input < -8.0 or > +16 Vdc is to be measured. Under no circumstances
should voltages greater than
±50 V be connected to channels C1 – C8.