Siemens SIMATIC S7 Functional Safety Manual page 66

Fail-Safe signal module (SM) applications
● The SM 1226 F-DQ 4 x 24 VDC provides ON test pulses to OFF switches, and OFF test
pulses to ON switches, to test that the P and M switches respond. The OFF test pulse
duration can be as long as your configured "Maximum readback time". The ON test pulse
duration can be as long as your configured "Maximum readback time switch on test". The
ON test pulse is only provided to one of the P or M switches at a time, but in the presence
of single fault this could apply energy to your load. Your actuator should not respond to
an OFF signal up to the "Maximum readback time" duration, or to an ON signal up to the
"Maximum readback time switch on test" duration.
● Both the SM 1226 F-DQ 4 x 24 VDC and SM 1226 F-DQ 2 x Relay expect to complete
and confirm any commanded transition from ON to OFF output state. After your program
changes process output value from "1" to "0", you should not change the "0" back to a "1"
until the F-DQ DC readback time or the F-RLY response time has completed. The
affected output passivates on commanded "0" states that are too short to confirm.
The safety performance of your installation depends on your design and continued
maintenance of each complete safety function.
The S7-1200 fail-safe CPU and fail-safe signal modules provide components for logic
processing with a certified level of safety integrity when used in accordance with ratings,
specifications, and instructions.
Failure to comply with these guidelines could cause damage or unpredictable operation
which could result in death or severe personal injury and/or property damage.
You must choose all components of your installation and complete your design and
maintenance according to accepted safety standards and practices to achieve your
required level of safety.
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WARNING
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA