Siemens SIMATIC S7-1500 System Manual page 43

Application planning
4.2 System and load power supply
Infeed via system power supply only
As a further possibility you can feed the required power into the backplane bus using only a
system power supply. In this case, the CPU is not supplied with 24 VDC, and draws its
supply from the backplane bus. Insert the system power supply next to the CPU, on the left.
In general you can use system power supplies with AC or DC infeed for the configuration. If
the first power segment is to be supplied directly with 230 VAC, an infeed via only one
system power supply makes sense, for example if no 24 VDC supply voltage is available.
Parameter assignment of the CPU: In STEP 7, select the "Connection to supply voltage L+"
option in the "Properties" tab in the "System power supply" area navigation so that STEP 7
can calculate the balance correctly.
Figure 4-6
42
No infeed into the backplane bus through the CPU
S7-1500 Automation System
System Manual, 02/2014, A5E03461182-AB