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SDM-SW8A Switch Closure Input Module
addressed SW8A is present. The 21X works similarly, except the data from
the SW8A is input to single ended analog channel 1, not C1.
Depending on the Command, Reps, and Channel information, the module will
shift out one or more bytes to the datalogger, again using C2 as a clock driven
by the datalogger. The module shifts out each bit on the falling edge of the
clock; the datalogger reads each bit on the rising edge of the clock.
Each time an entire byte is transmitted to the datalogger, the SW8A is
interrupted and prepares to send the next byte, if any. When all requested
bytes have been sent, the SW8A disables its serial communication interface
and waits for both C3 and C2 to be driven low by the datalogger. When this
happens, the SW8A prepares again to start a new command cycle.
An important feature of the module is its watchdog counter. The counter pulls
the processor momentarily into reset if the count gets too high. The counter
counts the C2 clock transitions. Under normal operating conditions, the
processor resets the counter. If the processor is "bombed," it will not reset the
counter. As the datalogger makes requests of the Switch Closure Module, the
counter increments to the point where it resets the processor; the module will
then start operating correctly again.
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