Description
Fail-safe SIMATIC output modules test their outputs with so-called bit pattern tests. The bit pattern test consists of a dark test and a light test:

Dark test
For test purposes a brief "0" signal is switched to the terminals of the F-DO in order, for example, to detect a short-circuit against the supply voltage.

Light test
For test purposes a brief "1" signal is switched to the terminals of the F-DO in order, for example, to detect a short-circuit against ground.

Now, if you interconnect a fail-safe input of a converter (in the following the SINAMICS G120) with a fail-safe output of a fail-safe SIMATIC output module, the converter can react to the signals of the bit pattern test (fault).

Solution (general)
The SINAMICS G120 has an adjustable signal filter to suppress brief signal changes of the bit pattern test.

Solution for S7-300 IO modules
You need P/P switching fail-safe outputs to address the fail-safe digital inputs of the SINAMICS G120. The IO module SM326 DO 10x24 V / 2 A PP from the S7-300 range fulfills this requirement. The filter on the SINAMICS G120 is to be enabled.
The figure below shows a sample interconnection of a converter and an SM326 IO module in a control cabinet.

Fig. 01

P/M-switching modules like the SM 326, F-DO 8 x DC 24V/2A PM (6ES7326-2BF41-0AB0) are not to be connected directly to the converter, but via interface relays.

Solution for ET 200S IO modules
You need P/P switching fail-safe outputs to address the fail-safe digital inputs of the SINAMICS G120. The EM 1 F-RO DC 24 V / AC 24...230 V / 5 A fail-safe relay module from the ET 200S system range fulfills this requirement. The fail-safe relay module is addressed via a fail-safe ET 200S output module.
The figure below shows a sample interconnection of a converter and an IO module in a control cabinet.

Fig. 02

As an alternative to the solution shown in the figure you can also use the PM E F pp fail-safe power module to address the fail-safe digital inputs of the SINAMICS G120.

Further information
More detailed information is available in the manuals below:

• SINAMICS G120 (Entry ID: 46253454),
• S7-300 Fail-safe Signal Modules (Entry ID: 19026151)
• ET 200S Distributed IO System (Entry ID: 34474892)

Keywords
Safety Integrated, Distributed Safety, Safety engineering, P-switching, M-switching, Light period, Dark period, Light and dark period, ET 200M