Siemens SIMATIC S7 System Manual page 45

priority group, the CPU then returns to the lower priority group and resumes the processing
of the pre-empted OB at the point where the processing of that OB had been interrupted.
Interrupt latency
The interrupt event latency (the time from notification of the CPU that an event has occurred
until the CPU begins execution of the first instruction in the OB that services the event) is
approximately 210 µsec, provided that a program cycle OB is the only event service routine
active at the time of the interrupt event.
Understanding time error events
The occurrence of any of several different time error conditions results in a time error event.
The following time errors are supported:
● Maximum cycle time exceeded
● Requested OB cannot be started
● Queue overflow occurred
The maximum cycle time exceeded condition results if the program cycle does not complete
within the specified maximum scan cycle time. See the section on "Monitoring the cycle time
(Page 43)" for more information regarding the maximum cycle time condition, how to
configure the maximum scan cycle time, and how to reset the cycle timer.
The requested OB cannot be started condition results if an OB is requested by a cyclic
interrupt or a time-delay interrupt, but the requested OB is already being executed.
The queue overflow occurred condition results if the interrupts are occurring faster than they
can be processed. The number of pending (queued) events is limited using a different queue
for each event type. If an event occurs when the corresponding queue is full, a time error
event is generated.
All time error events trigger the execution of OB 80 if it exists. If OB 80 does not exist, then
the CPU ignores the error. If two maximum cycle time exceeded conditions occur within the
same program cycle without resetting the cycle timer, then the CPU transitions to STOP,
regardless of whether OB 80 exists. See the section on "Monitoring the cycle time".
(Page 43)
OB 80 includes startup information that helps you determine which event and OB generated
the time error. You can program instructions inside OB 80 to examine these startup values
and to take appropriate action. The following startup locations are supported by OB 80:
Input
fault_id
csg_OBnr
csg_prio
No time error interrupt OB 80 is present when you create a new project. If desired, you add a
time error interrupt OB 80 to your project by double-clicking "Add new block" under "Program
blocks" in the tree, then choose "Organization block", and then "Time error interrupt".
S7-1200 Programmable controller
System Manual, 11/2009, A5E02486680-02
Data type Description
BYTE 16#01 - maximum cycle time exceeded
16#02 - requested OB cannot be started
16#07 and 16#09 - queue overflow occurred
OB_ANY Number of the OB which was being executed when the error
occurred
UINT Priority of the OB causing the error
PLC concepts
3.1 Execution of the user program
45