Siemens SINUMERIK 840D sl Function Manual page 1038

Detailed description
2.7 Structure and functions of the basic program
2.7.4 Time-alarm processing (OB 35)
General
The user must program OB 35 for time-alarm processing. The default time base setting of
OB 35 is 100 ms. Another time base can be selected using the STEP7 application "S7
Configuration". However, the OB 35 with a time base setting not less than approx. 15 ms
must not be used without additional measures, otherwise this would cause the PLC CPU to
stop. The stop is caused by reading of the system state list during power up of the HMI. This
reading process blocks priority class control for approx. 8 to 12 ms. The OB 35 with a time
base set to a rather lower value is then no longer processed correctly. If, however, small time
base settings are required for OB 35, the stop can be prevented by programming OB 80 with
at least the program command "BE".
2.7.5 Process interrupt processing (OB 40)
General
A process interrupt OB 40 (interrupt) can, for example, be triggered by appropriately
configured I/Os or by certain NC functions. Due to the different origin of the interrupt, the
PLC user program must first interpret the cause of the interrupt in OB 40. The cause of the
interrupt is contained in the local data of OB 40.
(For more information please also refer to the SIMATIC STEP7 description or the STEP7
online help).
2.7.6 Response to NC failure
General
During cyclic operation, the PLC continuously monitors NC availability by querying the sign-
oflife character. If the NCK is no longer reacting, then the NCK PLC interface is neutralized
and IS NCK CPU ready in signals from NC group (DB 10.DBX 104.7) is reset. The signals
sent by the NCK to the PLC are also set to an initial state.
The PLC itself remains active so that it can continue to control machine functions.
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Basic logic functions: PLC Basic program powerline (P3 pl)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0