Siemens SINUMERIK 840D sl Function Manual page 260

Connecting sensors and actuators
8.5 Safe programmable logic
This state occurs if data for crosswise data comparison is not exchanged between the NCK
and PLC for one second. This is due to the fact that
● the one second limit of the user cycle limit in the PLC (OB1 cycle) was exceeded.
● a system error has occurred. The NCK or PLC system software no longer runs due to a
system error - therefore interrupting communications.
Behavior of the NCK
The specified timer of 5 s is started if Alarm 27092 "Communication interrupted for crosswise
Data comparison NCK-SPL, error detected by %1" is initiated. This is independent of which
component (NCK or PLC) interrupted the alarm.
The system variable $A_STATSID, bit 29 = 1 is used to indicate to the SPL user that this timer
has been started. This means that he has a possibility of initiating plant/system-specific actions
before the system deletes (clears) the output.
After this time has expired, the system deletes the external SPL outputs. The status variable
$A_STATSID, bit 29 remains set. When reading-back the external outputs in the NCK-SPL via
the system variable $A_OUTSE, "0" is read corresponding to the actual output status.
Behavior of the PLC
If, on the PLC side, it is detected that the communication timeout has been exceeded, then a
timer is started with 5 s.
After this time has expired, the PLC goes into Stop (by calling an SFC46). This state can only
be exited using a power on.
After the 5 s timer has expired, for diagnostics purposes, the PLC outputs Alarm 400253 "PLC-
STOP due to SPL system error". At the same time, an entry with the same significance is in
the diagnostics buffer.
Using the status signal DB18.DBX119.5, the SPL programmer and the NCK is provided with
the information that the timer was started. This means that he has the possibility of initiating
system-specific actions before the PLC goes into the stop condition.
Note
In order to achieve the shortest possible response time, the system variable $A_STATSID, bit
29 and the status signal DB18.DBX119.5 must be evaluated in the SPL in order to bring, as
far as possible, the SPL-SGA into a safe state (cleared SPL-SGA).
Supplementary conditions
The user must take into consideration the following points regarding the functioning of the
crosswise data comparison:
● Both channels (NCK/PLC) must execute the same logic.
● Do not implement any response sequences or sequence controllers that are externally
controlled using short input pulses. This is because short pulses of this type may only be
sent and processed in one channel because of sampling effects.
● Unused inputs/outputs/markers of the SPL must be assigned the default value = 0; single-
channel use of individual bits for non-safety relevant purposes is not permissible.
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Function Manual, 12/2017, 6FC5397-4BP40-6BA1
Safety Integrated