Rockwell Automation 1769-PB4 Compact I/O Expansion Power Supplies - Manuel (Page 16)

16 Compact I/O Expansion Power Supplies
Publication 1769-IN028B-EN-P - October 2008
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 5 milliseconds…10 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. The processor then performs an orderly shutdown of the controller.
Input States on Power Down
The power supply hold-up time 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.
User Power Overcurrent Condition
The power supply shuts down in the event of an overcurrent condition. All outputs latch off
and remain off until you remove the overcurrent and cycle power. Reload the user program
following a power supply shutdown.
ATTENTION
To avoid unexpected operation due to 24V DC user power shutdown (1769-PA2 only),
monitor the 24V DC user output with a 24V DC input channel.