Siemens SIMATIC ET 200S Installation And Operating Manual page 73

Connection of Two Relays on One Digital Output
You can connect two relays using one fail-safe digital output. The following conditions should
be kept in mind:
• L+ and M of the relays must be connected with L+ and M of the PM-E F pm (the same
• The normally open contacts of the two relays must be connected in series.
This connection can only be made on digital outputs DO 0 and DO 1 (not DO 2). With this
circuit, you achieve:
• AK6/SIL3/Category 4 without standard DO modules on P1 and P2
• AK4/SIL2/Category 3 with standard DO modules on P1 and P2
Figure 7-7
ET 200S Distributed I/O System - Fail-Safe Modules
Hardware Installation and Operating Manual, 07/2005, A5E00103686-04
reference potential is necessary).
Wiring Diagram for Each of Two Relays on DO 0 and DO 1 of the PM-E F pm DC24V
PROFIsafe
Warning
When connecting two relays to one digital output (as shown above), the "wire break" and
"overload" faults are only detected at the output P-switch (not at the M-switch).
Warning
In the event of a cross circuit between the output P and M-switch the controlled actuator
is no longer switched off. To avoid cross circuits between P and M-switches of a fail-safe
digital output, you should connect the relay on the between P and M-switches to protect
against cross circuits (for example with cables separately sheathed or in a separate cable
duct).
7.2 PM-E F pm DC24V PROFIsafe Power Module
Fail-Safe Modules
7-13