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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