Fail-Safe signal module (SM) applications
3.2 Digital output applications
S7-1200 Functional Safety Manual
74 Manual, 02/2015, A5E03470344-AA
Digital output applications
You should consider the application modes presented here along with the features of the SM
1226 F-DQ 4 x 24 VDC and SM1226 F-DQ 2 x Relay as described in the overview. Refer to
"SM 1226 F-DQ 4 x 24 VDC" (Page 21) and "SM 1226 F-DQ 2 x Relay" (Page 23).
These are the main features to keep in mind for the SM1226 F-DQ 4 x 24 VDC:
● The F-DQ DC separately controls current flow in the P (24 V to load) and M (load to
common) sides of the circuit.
● The voltage at P and M outputs is read back to confirm proper state.
● The P and M switches are regularly tested with short ON and OFF pulses to confirm
control.
● You must configure readback times that allow the external voltage to respond but do not
cause your load to physically respond.
● Internal current limiting as described in the data sheet may be sufficient in combination
with your 24 VDC supply, but you must consider whether additional current limits or fuses
are necessary. Refer to Appendix A.3.3.3: "Specifications" (Page 178) (SM1226 F-DQ 4 x
24 VDC).
These are the main features to keep in mind for the SM1226 F-DQ 2 x Relay:
● The F-RLY controls each circuit with two independently-controlled series contacts.
● The series contacts are sequenced to avoid welding as a common cause failure.
● All internal relays include mechanically-linked sense contacts with read back.
● You must configure a maximum ON time for each channel, effectively the time between
tests that the channel can be turned OFF. For SIL 3 applications, this time must be 30
days or less.
● There are two complete circuits for each process channel.
● The two circuits on each channel must be the same voltage category: both SELV/PELV
or both line voltage
● Each circuit must be protected by an external fuse as defined in the data sheet.
All output channels are controlled as 1oo2 with cross-diagnostics.
All applications shown are capable of reaching SIL 3/Category 4/PL e.
To reach Category 4, external contactors must be SIL-rated with sense contacts. You must
read the sense contacts back and confirm the external relay response in your program.
Siemens recommends using an F-DI input for sense contact and other safety diagnostic
inputs.
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