Siemens SINUMERIK 840Di Function Manual page 960

Detailed description
2.7 Structure and functions of the basic program
2.7.3 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 the reading of the MMC system state list during runup of the
MMC. 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".
2.7.4 Process-alarm processing (OB 40)
General
A process alarm 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.
2.7.5 Response to NC failure
General
During cyclic operation, the PLC continuously monitors NC availability by querying the sign
of life 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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Power Line Basic PLC Program (P3)
Function Manual, 08/2005 Edition, 6FC5397-0BP10-0BA0