Siemens SIMATIC Operating Instructions

Fail-Safe Electronic Modules
8.2 4 /8 F-DI/4 F-DO DC24V/2A PROFIsafe Digital Electronic Module
ET 200pro Distributed I/O System - Fail-Safe Modules
98 Operating Instructions, 05/2007, A5E00394073-02
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
potential must be equal).
● 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+ M
DO0
(M0)
DO0
(P0)
DO1
(M1)
DO1
(P1)
DO2
(M2)
DO2
(P2)
DO3
(M3)
DO3
(P3)
4/8 F-DI/4 F-DO
L+ M
K1
K1 K2
K2
M
2L+
Figure 8-19 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
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.
WARNING
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).

Siemens SIMATIC Specifications

General

Product type designation	SIMATIC
CPU	Varies by model
Memory	Varies by model
Input/Output	Varies by model
Communication	Varies by model
Programming Language	STEP 7
Operating Temperature	0°C to 60°C (varies by model)
Protection Class	IP20
Category	Programmable Logic Controller (PLC)