Siemens SIMATIC S7 Functional Safety Manual page 22

Product overview
1.4 S7-1200 Fail-Safe signal modules (SM)
Outputs
The F-DQ DC consists of four output channels (F-DQ a.0...F-DQ a.3). You can use each
output for SIL 3 applications. Each output consists of two switches:
● A P-switch connects 24V positive (L+) to the load.
● An M-switch connects the load to M or 24V return.
Both switches must be turned ON for current to flow to the load.
The F-DQ DC uses two microcomputers to implement the safety function. One
microcomputer controls the P-switch while the other microcomputer controls the M-switch.
There is feedback from the P-switch output to the other microcomputer that is controlling the
M-switch. Likewise, there is feedback from the M-switch output to the other microcomputer
that is controlling the P-switch. The feedback verifies that the output switches are operating
properly and in the commanded state.
You must configure a "Maximum readback time" that specifies the allowed delay for the
output voltage to respond to the switch change.
The F-DQ DC regularly tests each "OFF" switch "ON" briefly, and each "ON" switch "OFF"
briefly, to verify that each switch is still functional and under independent control. Your
configured "Maximum readback time" also sets the duration of the "OFF" test pulse. You
must configure a "Maximum readback time switch on test" which sets the duration of the
"ON" test pulse. You should choose these durations short enough to not affect your load.
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S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA