Rockwell Automation 1747-ACNR15 ControlNet Adapter Module User Manual - Manual (Page 108)

Publication 1747-UM003A-EN-P
B-16 Understanding Your SLC 500/1746 Control System
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 “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 between 20 ms and
3 s. When the duration of power loss reaches this limit, the power supply
signals can no longer provide adequate dc power to the system. This is referred
to as a power supply shutdown. The power supply LED is turned off.
In multi-chassis systems, power outages of 20 to 300 ms in duration can cause
a power fail error to occur. You can clear this error by cycling power to your
system.
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 1747-ACN15/-ACNR15 and sent to 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. 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.
Other Types of Line Conditions
Occasionally the power source to the system can be temporarily interrupted. It
is also possible that the voltage level drops 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.
Power Conditioning Considerations
There are two types of power conditioning considerations: isolation and
suppression.