Time Error Interrupt Ob - Siemens SIMATIC S7-1200 System Manual
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5.1 Execution of the user program
5.1.3.6
Time error interrupt OB
If configured, the time error interrupt OB (OB 80) executes when either the scan cycle
exceeds the maximum cycle time or a time error event occurs. If triggered, it executes,
interrupting normal cyclic program execution or any other event OB.
The occurrence of either of these events generates a diagnostic buffer entry describing the
event. The diagnostic buffer entry is generated regardless of the existence of the time error
interrupt OB.
Time error interrupt events
The occurrence of any of several different time error conditions results in a time error event:
● Scan cycle exceeds maximum cycle time
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 in the S7-1200 System Manual" (Page 109) for more information regarding
the maximum cycle time condition, how to configure the maximum scan cycle time in the
properties of the CPU, and how to reset the cycle timer.
● CPU cannot start requested OB because a second time interrupt (cyclic or time-delay)
starts before the CPU finishes execution of the first interrupt OB
● Queue overflow occurred
The "queue overflow occurred" condition results if the interrupts are occurring faster than
the CPU can process them. The CPU limits the number of pending (queued) events by
using a different queue for each event type. If an event occurs when the corresponding
queue is full, the CPU generates a time error event.
All time error events trigger the execution of the time error interrupt OB if it exists. If the time
error interrupt OB does not exist, then the device configuration of the CPU determines the
CPU reaction to the time error:
● The default configuration for time errors, such as starting a second cyclic interrupt before
the CPU has finished the execution of the first, is for the CPU to stay in RUN.
● The default configuration for exceeding the maximum time is for the CPU to change to
STOP.
The user program can extend the program cycle execution time up to ten times the
configured maximum cycle time by executing the RE_TRIGR instruction (Page 309) to
restart the cycle time monitor. However, 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 the time error interrupt OB exists. See the section
on "Monitoring the cycle time in the S7-1200 System Manual" (Page 109).
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S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
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