Siemens Sinumerik 840D sl Function Manual page 196

Connecting Sensors/Actuators
7.1 Safety- -relevant input/output signals
S External inputs/outputs
The external inputs/outputs of the SPL ($A_INSE or $A_OUTSE) must be sub-
ject to a forced checking procedure to ensure that faults do not accumulate over
a period of time which would mean that both monitoring channels could fail.
S Internal inputs/outputs
Internal inputs/outputs ($A_INSI, $A_OUTSI), markers ($A_MARKERSI) etc.
($A_TIMERSI) do not have to be subject to a forced checking procedure. It will
always be possible to detect an error at these locations due to the differing two--
channel responses of the external inputs/outputs or the NCK/drive monitoring
channels; crosswise data comparison is carried out at both ends of the re-
sponse chain to detect any errors.
Test signals
"3- -terminal concept" (see Chapter 7.1.3 "Connecting sensors -- actuators using
the 3--terminal concept"):
S If an input signal ($A_INSE)is, for example, evaluated through two channels,
the associated test output signal can be implemented using one channel. It is
extremely important that the input signal can be forced/changed and checked in
both channels.
S In the same way, the assigned test input signal for two--channel output signals
($A_OUTSE) can be implemented in one channel if it is connected according to
the following rule:
The test input signal may only return an "OK" status ("1" signal level) if both
output signals function (i.e. both monitoring channels have output a "0").
A simultaneous test in both channels allows the correct functioning in both
channels to be checked using one feedback signal.
Trigger/test
The timer or event controlled triggering of the forced checking procedure is activa-
ted in one channel by the PLC.
If errors are detected, the PLC user program should respond by initiating an exter-
nal "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 net (useful) 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 admissible:
S A "full 4--terminal concept for sensors" (two--channel test signal for a two--
channel useful [net] signal),
S the "3--terminal concept for sensors/actuators" recommended above
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SINUMERIK 840D sl/SINAMICS S120 SINUMERIK Safety Integrated (FBSI sl) - - 10.2015 Edition
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