Campbell Scientific CR510 Basic Datalogger - Manual (Page 106)

SECTION 9. INPUT/OUTPUT INSTRUCTIONS
9-2
volts to above 3.5 volts. The maximum input
voltage is +20 volts. A problem, however,
arises when the pulse is actually a low
frequency signal (below about 10 Hz) and the
positive voltage excursion exceeds 5.6 VDC.
When this happens, the excess voltage is
shunted to the CR510 5 VDC supply, with the
current limited by an internal 10 Kohm resistor.
When this extra current source exceeds the
quiescent current needs of the CR510 (about
0.6 mA), the 5 VDC supply will start to rise,
upsetting all analog measurements.
Thus, pulses whose positive voltage portion
exceeds 5.6 VDC with a duration longer than 100
milliseconds need external conditioning. One
method would be to use a 4 to 5.6 V zener diode
from the signal to ground. The simplest method,
however, is to add an external 20 Kohm resistor in
series with the signal (Figure 9-1). This will limit
the current for pulses to 20 VDC to the point that it
will not upset the CR510 5 VDC supply.
LOW LEVEL AC
This configuration is used to count the frequency of
AC signals from magnetic pulse flow transducers
or other low voltage, sine wave inputs. The
minimum input voltage is 6 millivolts RMS. Input
hysteresis is 11 millivolts. The maximum AC input
voltage is 20 volts RMS. The maximum input
frequency ranges from 1000 Hz at 20 mV RMS to
16,000 Hz at 1000 mV or greater.
SWITCH CLOSURE (Pulse Inputs)
In this configuration, the minimum switch closed
time is 5 milliseconds. The minimum switch open
time is 6 milliseconds. The maximum bounce
time is 1 millisecond open without being counted.
The switch is connected between the pulse input
and ground (G). When the switch is open, the
CR510 pulls the pulse input to 5 volts. When the
switch is closed, the pulse input is at ground. The
count is incremented when the switch opens.
SWITCH CLOSURE (Control Ports)
Control Ports C1, C2/P3 can be used to
measure switch closures < 40 Hz. The minimum
switch opened or closed time is 6 milliseconds.
The switch is connected between the control
port and 5 V. When the switch is open, the
control port is pulled to ground by an internal
100 Kohm resistor. When the switch is closed,
the control port is at 5 V. The count is
incremented when the switch closes.
NOTE: If Control Port C2/P3 is used for
pulse measurements or interrupt
subroutines, the CR510 will not go into the
quiescent power state (0.7 mA) if Control
Port C2/P3 is high.
PULSE MEASUREMENT DETAILS
The 2 pulse count input channels each have
eight bit counters. Input frequencies greater
than 2000 Hz (the limit of the eight bit counter,
255 counts at the reset interval of 0.125
second) can be counted by combining two
counters on one input channel. When this
option is selected, channel 1 is used for the
pulse input. Channel 2 is not used.
CR510 CR510
FIGURE 9-1. Conditioning for Long Duration Voltage Pulses