Siemens SIMATIC S7-1500 Manual page 36

Applications of the F-I/O module
5.3 Application: Connection of loads per digital output to L+ and M, PM-switching mode
When two relays are connected to one digital output, as shown in the figure above, the
"wire break" and "overload" faults are detected only at the P-switch of the output (not at the
M-switch).
With a cross circuit between P-switch and M-switch of the output, the module detects the
fault and switches off the output. But the actuator is still supplied with power due to the
external fault.
To avoid cross circuits between the P and M-switches of a fail-safe digital output, you
should always wire the relay connection to the P and M-switches separately, in order to
prevent any cross circuits. For example, as separately sheathed cables or in separate cable
ducts.
Note
The digital output module F-DQ 8x24VDC/2A PPM performs a bit pattern test depending on
the parameter assignment. For this, the module outputs up to 5 ms pulses depending on the
parameter assignment. This test (switch-on test) is run with a time offset between the P-
switch and M-switch to prevent the actuator from being activated. This pulse may cause the
corresponding relay to operated, which may reduce its mission time.
We therefore recommend adhering to the wiring scheme described below. See section
Application: Connecting 2 loads in parallel per digital output, PM-switching mode (Page 37).
Parameter assignment
Assign the following parameter for the corresponding channel group:
Table 5- 2
Parameter
Operating mode of the output
36
WARNING
WARNING
Parameter
PM-switching mode
Digital output module F-DQ 8x24VDC/2A PPM (6ES7526-2BF00-0AB0)
Manual, 01/2016, A5E03858037-AA