Campbell Scientific SDM-SW8A 8-Channel Switch Closure Input Module - Manual (Page 17)

SDM-SW8A Switch Closure Input Module
8. Measurement Applications
8.1 SPDT Switch Closure
Single-pole double-throw switches may be found on some flow or volume
sensors such as Watt-hour and water meters. The positive throw is connected
to the 5 V terminal located next to the input channel, providing a 5 V bias for
the SW8A to discriminate between throws. Similarly, the negative throw is
connected to a ground terminal (refer to Figure 2, SPDT wiring example).
When contact is made to the positive throw, SW8A circuitry holds a high state
(5 V). When contact is made with the ground throw, a low state is maintained
(0 V). Switch bounce may occur any number of times at a throw, but until
contact is made with the opposite throw, a change in state will not occur. The
pole must make contact with the throw for 3 ms for a state change to occur.
Figure 3 illustrates a raw SPDT signal in relation to the signal conditioned by
the SW8A.
FIGURE 3. SPDT Signal Conditioning by SDM-SW8A
The 5V output located next to each of the 8 input channels is for
biasing in the SPDT measurement. A 200 Ohm resistor is in
series to protect against accidental shorting to ground. NOTE
8.2 SPST Switch Closure
Single-pole single-throw switches are either open (high state) or connected to
ground (low state). Typical SPST switches include contact closure (reed
switch) anemometers, tipping bucket rain gauges, and open collectors
(semiconductor switches). Many control devices utilize open collectors and
provide terminals for monitoring the switch.
For all SW8A measurements, the 100 Hz maximum input frequency and 5 ms
pulse width specifications must be taken into consideration. To illustrate,
consider the Met-One 014A Cup Anemometer which is an SPST-type sensor.
The 014A calibration is:
MPH = 1.789 * f + 1
where: MPH = miles per hour
f = pulse frequency in Hz
The 100 Hz maximum input frequency to the SW8A equates to 180 MPH.
The duty cycle of the 014A as measured by the SW8A is 35%. A 5 ms pulse
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