Siemens SIMATIC S7 Functional Safety Manual page 92

Fail-Safe CPU and signal module (SM) installation
4.2 Fail-Safe system electrical design rules
Refer to the technical specifications for information about power input requirements (CPUs,
SMs, SBs, and CMs) and CPU power output limits (5 VDC and 24 VDC).
Refer to the "Calculating a power budget" (Page 94) to determine CPU power output
capability.
Expansion module installation rules:
● Standard and fail-safe signal modules (SM) are installed on the right side of the CPU.
Fail-safe CPUs allow a maximum of 8 signal modules, if the total of all add-on module
CPU loads does not exceed the CPU's 5 VDC and 24 VDC power supply load limits. Fail-
safe and standard modules may be intermixed on the right side of the CPU.
● Communication modules (CM), are installed on the left side of the CPU. A maximum of 3
communication modules is allowed for a Fail-safe CPU, if the total of all add-on module
CPU loads does not exceed the CPU's 5 VDC and 24 VDC load limits.
● Signal boards (SB), communications boards (CB), and battery boards (BB) are installed
on top of the CPU. A maximum of one signal board, communication board, or battery
board is allowed for any CPU.
Do not connect separate power supplies directly in parallel.
This can cause a conflict between the power supplies, as each supply tries to establish its
preferred output voltage level.
The result of this conflict can be shortened lifetime or the immediate failure of one or both
power supplies, with consequent unpredictable operation of the PLC system. Unpredictable
operation could result in death, severe personal injury and property damage.
A CPU 24 VDC supply and any external power supply should provide power to different
load points.
For improved electrical noise protection, the commons (M) of the different power supplies
should be connected.
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WARNING
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA