Siemens SINUMERIK 840D sl Function Manual page 178

Connecting sensors and actuators
8.1 Safety-related input/output signals
Trigger/test
The timer or event controlled triggering of the forced checking procedure is activated in one
channel by the PLC.
If errors are detected, the PLC user program should respond by initiating an external "STOP
D/E" and switching the external SGAs into a safe state.
Notes to avoid errors
1. A "2-terminal concept" in which a single-channel user (net) signal is to be subject to a forced
checking procedure using a single-channel test signal is not permitted. In this case, the two-
channel SPL structure would be worthless and crosswise data comparison would have no
effect.
The following are permissible:
– A "full 4-terminal concept for sensors" (two-channel test signal for a two-channel user
– the "3 terminal concept for sensors/actuators" recommended above
– a "2-terminal concept for sensors without test signals", if the sensor is a safety-related
– a "2-terminal concept for actuators without test signals", if the actuator is a safety-related
– a "2-terminal concept for actuators without test signals" if the feedback signal can be
– a "2-terminal concept for actuators without test signals" if the function of the mechanical
2. The signals "external STOPs" are processed internally in a special way:
– In order to increase the level of reliability that a requested "external STOP" actually takes
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(net) signal),
a "2-terminal concept for sensors without test signals", if the two-channel user (net)
signal to be tested automatically changes its level dynamically as a result of the process.
For example, for the input signals of a protective door,
component, e.g. light curtains,
component, e.g. safety valves,
checked using other user (net) signals. For example, for a valve that indirectly switches
a BERO via the process and this is available for evaluation,
system can be checked using other user (net) signals. For example, for a brake that is
checked using a separate brake test.
effect, the STOPs are internally exchanged between the two channels. Failure of the
stop control function in one channel does not cause an error for these signals (in contrast
to the mode changeover signals, e.g. "SG/SBH active") in the crosswise data
comparison.
While other signals can be subject to a forced checking procedure in parallel and in both
channels (in order to avoid errors being triggered by the crosswise data comparison),
the "external STOPs" must be subject to a forced checking procedure one after the other
in both channels.
Function Manual, 12/2017, 6FC5397-4BP40-6BA1
Safety Integrated