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115 VAC 8 Circuit Grouped I/O Blocks perform the diagnostics described below. A block reports all faults to the Hand–held Monitor, and takes appropriate corrective action. Individual circuits can be configured not to send diagnostic messages to the CPU if a fault occurs. If the CPU requests diagnostic information from the block using Read Diagnostics datagrams, the block returns current diagnostics for all circuits, including any with CPU fault reporting disabled. Overtemperature Diagnostic Each circuit has a built–in thermal sensor. If the block’s internal temperature exceeds 100C, the block sends an OVERTEMPERATURE message and turns off the circuit to protect its internal electronics. This diagnostic is always performed for both inputs and outputs. Short Circuit Diagnostic Automatic output diagnostic. Output circuits are protected by a short circuit level sensor at the switching device. If the instantaneous current on an output exceeds 30 amps during the first two line cycles or 20 amps thereafter, the block turns the output off within microseconds. The block will try to restart the load; if several attempts are unsuccessful, the output circuit is forced off and the block sends a SHORT CIRCUIT message. To restore normal operation to output the cause of the current surge must be removed, then the diagnostic must be cleared from the HHM or the CPU. Failed Switch Diagnostic The block automatically monitors all circuits for several types of faults, which may be reported as Failed Switch diagnostics. For an output, Failed Switch is reported if the circuit’s switch state is not the same as its commanded state. The block sends a FAILED SWITCH message identifying the failed circuit. The logic state of the circuit is set to OFF. When an output fault occurs, the actual condition of the output switch is not known. If the output switch has failed shorted (or closed), current flow is not interrupted when the block forces the output state OFF. Action external to the block must be taken to remedy the problem. The FAILED SWITCH message can alert personnel or cause program logic to be activated, possibly shutting off power to the block, I/O section, or process.