Siemens S7-300 Installation And Operating Manual page 132

Digital modules
8.5 SM 326; DO 8 x DC 24V/2A PM
Wiring two relays to one digital output
The fail-safe digital output is capable of controlling two relays. Note the following conditions:
● Wire L+ and M of the relays to L+ and M of the module (same reference potential
● Wire the NO contacts of the two relays in series.
The relays can be wired to any one of the 8 digital outputs. The figure below shows an
example of an output wiring. This circuit is compliant with SIL 3/Cat. 4/PLe.
Figure 8-38
To prevent short circuits between the P and M switches of a fail-safe digital output, you
must route the cables used to connect the relays on the P and M switches in a cross-circuit-
proof manner (e.g., as separate, unsheathed cables or in separate cable ducts).
The module only detects "wire break" and "overload" errors at the P switch (not at the M
switch) of a digital output which drives two relays.
It is no longer possible to shut down an actuator if a cross-circuit has developed between
the P and M switches of the output.
Note
The SM 236; DO 8 x DC 24V/2A PM carries out a bit pattern test ca. every 15 minutes. For
this purpose, the module sends out a pulse for max. 4 ms. The module runs the test with a
time offset between the P and M switches to prevent the actuator from being activated. This
pulse may cause the corresponding relay to tighten, which may reduce its service life.
We therefore recommend adhering to the wiring scheme detailed below.
132
required).
Wiring scheme 2 relays at digital output of SM 326; DO 8 x DC 24V/2A PM
WARNING
WARNING
Installation and Operating Manual, 01/2010, A5E00085586-10
Fail-safe signal modules