Siemens SIMATIC Operating Instructions Manual page 98

Fail-Safe Electronic Modules
8.2 4 /8 F-DI/4 F-DO DC24V/2A PROFIsafe Digital Electronic Module
Use Case 2: Wiring Loads to L+ and M at Each Digital Output
You can switch two relays with 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 F-DO module (reference
● The normally open contacts of the two relays must be switched in series.
A connection to each of the four digital outputs is possible. The following figure shows an
example of the connection at DO 0. This circuit achieves SIL3/Category 4.
2L+
2L+ M
L+
Figure 8-19
When two relays are connected at one digital output (as shown in the figure above), "wire
break" and "overload" faults are detected only at the P-switch of the output (not at the M-
switch).
The controlled actuator can no longer be switched off in the event of 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 must
route the cables for the relay connection at the P- and M-switches to protect against cross
circuits (e.g., as separately sheathed cables or in separate cable ducts).
98
potential must be equal).
DO0 DO0
(P0) (M0)
K1 K2
M
Wiring Diagram for Each of Two Relays at One F-DO of the 4/8 F-DI/4 F-DO DC24V/2A
PROFIsafe Electronic Module
WARNING
WARNING
4/8 F-DI/4 F-DO
DO1 DO1 DO2
(P1) (M1) (P2)
ET 200pro Distributed I/O System - Fail-Safe Modules
DO2 DO3 DO3
(M2) (P3) (M3)
K1
K2
M
Operating Instructions, 05/2007, A5E00394073-02