Siemens SIMATIC ET 200S Installation And Operating Manual page 74

Fail-Safe Modules
7.2 PM-E F pm DC24V PROFIsafe Power Module
Note
The PM-E F pm DC24V 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 7-8
Note
With parallel connection of two relays to one digital output (as shown above), the "wire
break" fault is only detect if the wire break disconnects both relays from P or M. This
diagnosis is not safety-relevant.
7-14
Wiring Diagram for Each of Two Relays on DO 0 and DO 1 of the PM-E F pm DC24V
PROFIsafe
Hardware Installation and Operating Manual, 07/2005, A5E00103686-04
ET 200S Distributed I/O System - Fail-Safe Modules