Monitoring And Configuring The Cycle Time - Siemens S7-1200 System Manual

In addition, the CPU recognizes other events that do not have associated OBs. The following
table describes these events and the corresponding CPU actions:
Table 4- 14 Additional events
Event
I/O access error
Max cycle time error
Peripheral access error I/O error during process image update
Programming error
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 175 µsec, provided that a program cycle OB is the only event service routine
active at the time of the interrupt event.
4.1.4

Monitoring and configuring the cycle time

The cycle time is the time that the CPU operating system requires to execute the cyclic
phase of the RUN mode. The CPU provides two methods of monitoring the cycle time:
● Maximum scan cycle time
● Minimum scan cycle time
Scan cycle monitoring begins after the startup event is complete. Configuration for this
feature appears under the "Device Configuration" for the CPU under "Cycle time".
The CPU always monitors the scan cycle and reacts if the maximum scan cycle time is
exceeded. If the configured maximum scan cycle time is exceeded, an error is generated
and is handled one of two ways:
● If the user program does not include a time error interrupt OB, then the timer error event
● If the user program includes a time error interrupt OB, then the CPU executes it.
The RE_TRIGR instruction (Page 262) (re-trigger cycle time monitoring) allows you to reset
the timer that measures the cycle time. If the elapsed time for the current program cycle
execution is less than ten times the configured maximum scan cycle time, the RE_TRIGR
instruction retriggers the cycle time monitoring and returns with ENO = TRUE. If not, the
RE_TRIGR instruction does not retrigger the cycle time monitoring. It returns ENO = FALSE.
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
Description
Direct I/O read/write error
CPU exceeds the configured cycle time
twice
program execution error
generates a diagnostic buffer entry, but the CPU remains in RUN mode. (You can change
the configuration of the CPU to go to STOP mode when it detects a time error, but the
default configuration is to remain in RUN mode.)
4.1 Execution of the user program
CPU action
The CPU logs the first occurrence in the diagnostic
buffer and stays in RUN mode.
The CPU logs the error in the diagnostic buffer and
transitions to STOP mode.
The CPU logs the first occurrence in the diagnostic
buffer and stays in RUN mode.
If the block with the error provides error handling, it
updates the error structure; if not, the CPU logs the
error in the diagnostic buffer and stays in RUN
mode.
PLC concepts
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