Siemens SIMATIC ET 200S Installation And Operating Manual page 147

Note
The EM 4 F-DO DC24V/2A PROFIsafe performs a bit pattern test approximately every
15 minutes. The module generates a max. 4 ms pulse for this. This test is performed time
staggered between the P and M switch, so that the actuator is not activated. However, the
pulse may be picked up by respective relay, which can result in a reduced service life for the
relay.
We therefore recommend the wiring scheme described in the following.
Avoiding/Dealing with Cross Circuits
To protect against cross circuits between P and M-switches of a fail-safe digital output, we
recommend the following wiring schemes:
Figure 7-39
Note
With a parallel connection of two relays on one digital output (as shown above) the "wire
break" fault is only detected if the wire break disconnected both relays from P or M. This
diagnosis is not safety-relevant.
ET 200S Distributed I/O System - Fail-Safe Modules
Hardware Installation and Operating Manual, 07/2005, A5E00103686-04
7.6 4 F-DO DC24V/2A PROFIsafe digital electronic module
Wiring diagram per two relays parallel on 1 F-DO of the EM 4 F-DO DC24V/2A
PROFIsafe
Fail-Safe Modules
7-87