Rockwell Automation 1747-L5xx SLC 500 Modular Hardware Style User Manual - Manual (Page 70)

Publication 1747-UM011G-EN-P - June 2008
70 System Installation Recommendations
Loss of Power Source
The chassis power supplies are 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
number, type, and state of the I/O modules, but is typically 20 ms...3 s. When
the duration of power loss reaches a 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 power supply status indicator is
turned off.
In multi-chassis systems, power outages of 20...300 ms in duration can cause a
remote power fail error to occur. You can clear this error by cycling power to
your system or by using a programming device.
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 input modules. 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. Write the user program to take this
effect into account. For example, hard-wire power to one spare input. In the
user program, check to be sure that one input is on; otherwise, jump to the end
of the program and avoid scanning the logic. Use of a common power source
as recommended in the previous section is assumed.
Power Supply Undervoltage Operation
SLC 500 controllers continue to operate (hold-up) for a short period of time if
the input voltage to the power supply drops below the recommended
operating voltage range. The controller continues to scan the user program
and control I/O during this time. CPU hold-up time is 20 ms...3 s depending
on the power supply and loading.
SLC 500 controllers turn OFF (stop scanning and disable outputs) if input
voltage to the power supply is removed or drops below the recommended
operating range for a period exceeding the CPU hold-up time. The controller
resumes operation automatically when the input voltage is restored to normal.
If the input voltage to the 1746-P7 power supply falls into a range of 4...9V for
a period exceeding the CPU hold-up time, the controller turns OFF and will
not turn back ON until the input voltage is increased to 11V dc.