Hardware
Preface
MicroLogix 1000 Programmable Controllers User Manual
1–14
Loss of Power Source
The power supply is designed to withstand brief power losses without affecting the
operation of the system. The time the system is operational during power loss is
called “program scan hold-up time after loss of power.” The duration of the power
supply hold-up time depends on the type and state of the I/O, but is typically
between 20 milliseconds and 3 seconds. When the duration of power loss reaches
this limit, the power supply signals the processor that it can no longer provide
adequate dc power to the system. This is referred to as a power supply shutdown.
Input States on Power Down
The power supply hold-up time as described above is generally longer than the
turn-on and turn-off times of the inputs. Because of this, the input state change from
“On” to “Off” that occurs when power is removed may be recorded by the processor
before the power supply shuts down the system. Understanding this concept is
important. The user program should be written to take this effect into account.
Other Types of Line Conditions
Occasionally the power source to the system can be temporarily interrupted. It is
also possible that the voltage level may drop substantially below the normal line
voltage range for a period of time. Both of these conditions are considered to be a
loss of power for the system.
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