Siemens SIMATIC ET 200S Installation And Operating Manual page 146

Fail-Safe Modules
7.6 4 F-DO DC24V/2A PROFIsafe digital electronic module
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 (reference
• The normally open contacts of the two relays must be connected in series.
A connection to each of the four digital outputs is possible. The following figure shows the
example of the connection at DO 0. This circuit achieves AK6/SIL3/Category 4.
Figure 7-38
Warning
When connecting two relays on one digital output, (as shown in the figure above), the errors
"wire break" and "overload" are detected only at the P-switch (not at the M-switch).
Warning
The controlled actuator can no longer be switched off when there is a cross circuit between
the P and M-switches of the output. 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).
7-86
potential must be equal).
Wiring diagram per two relays on 1 F-DO of the EM 4 F-DO DC24V/2A PROFIsafe
ET 200S Distributed I/O System - Fail-Safe Modules
Hardware Installation and Operating Manual, 07/2005, A5E00103686-04