Rockwell Automation 1747-L40 SLC 500 Fixed Hardware Style - Manual (Page 46)

Chapter 2
System Installation Recommendations
2–7
The following explains power considerations for the SLC 500 fixed
controller.
Common Power Source
We strongly recommend that the chassis power supply has the same power
source as the input and output devices. This helps:
•
reduce the chance of electrical interference due to multiple sources and
grounds
•
maintain system integrity if power is interrupted
Loss of Power Source
The chassis 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 number, type
and state of the I/O, but is typically between 20 ms and 700 ms. 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 LED is turned off.
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 circuits. 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. 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 Considerations