Campbell Scientific CR3000 Micrologger - Manual (Page 321)

Section 8. Operation
321
8.1.5.1.2 Low-Level ac (P1 - P4)
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 P4 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 - P4)
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. 562)
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 CR3000 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.
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 –
P4 (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.