Installation and WiringFP2/FP2SH
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4.6 Safety Measures
4.6.2 Momentary Power Failures
If the duration of the power failure is less than 10 ms, the FP2/FP2SH continues to oper-
ate. If the power is off for 10 ms or longer, operation changes depending on the
combination of units, the power supply voltage, and other factors. (In some cases, op-
eration may be the same as that for a power supply reset.)
4.6.3 Alarm Output
The alarm output goes on when the watchdog timer is activated by a program error or
an error in the hardware itself.
The alarm output terminal has two relay contacts, N.O. (normally open) and N.C. (nor-
mally closed). This can be used as an external alarm signal when an error occurs.
Lamp for alarm
signal
L
Power supply unit
NC
COM
NO
Watchdog timer
The watchdog timer is a program error and hardware error detection timer. It goes on
when the scan time exceeds 640 ms.
When the watchdog timer is activated, at the same time the ALARM LED lights, the
ALARM contacts on the power supply unit go on, all outputs to the output units are
turned off and the unit is put in halted state. The system is in a non-processing state that
includes communications with programming tools as well.
Note
The ALARM contacts of the power supply unit installed on the
expansion backplane will not also operate.
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