User manual

Publication 1747-UM011F-EN-P - May 2007
72 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.