SDM-SW8A Switch Closure Input Module

forces a watchdog reset by addressing the Module and requesting sufficient information to cause a minimum of 65 clock line transitions. Advantages to the trapping routine are:

•The bombed processor is detected before erroneous values (-99999) are included in subsequent processing.

•The processor may be reset sooner.

•The time of processor failure may be logged by the datalogger.

•Number of failures may be logged.

A trapping routine, as described above, is included in the example program in the Appendix.

8.Measurement Applications

8.1SPDT 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

NOTE 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.

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