Siemens S5-155U System Manual page 306

Technical Description
With enable input switched off If removal and insertion of modules during operation is not required,
Examples of functioning of the
enable inputs
8 - 8
you must remove the plug-in jumper for changeover of the enable
mode. Wiring of the enable inputs (F+/F-) can then be dispensed with.
Almost power-free shutdown of individual subprocesses, i.e. outputs
of various modules can be operated from a common load supply and
yet activated separately.
The load voltage of each individual module can be monitored without
additional circuitry. Any reactions to failure of the load voltage can be
programmed in the QVZ (timeout) organization block.
You must observe the following when configuring systems:
Switching on At the latest 100 ms after power-up of the PLC, the
voltage must be present at the enable inputs of the
I/O modules.
Switching off When the PLC has been switched off, the voltage
at the enable inputs of the I/O modules must still
be present as long as voltage is applied to the
CC/EU.
You should observe the following instructions for switching off PLCs
and equipment for supplying power to the enable inputs:
24 V supply for CC/EU and I/Os
CC/EU
24 V Power Supply
24 V DC
Enable supply from:
a) Battery
b) Terminals for 24 V on the front plate of the power supply
a)
Battery
b)
I/O Modules
F +
L +
L +
System Manual
C79000-B8576-C199-03