35 BASIC CONCEPT
35.10 Running a Safety Program
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Safety program execution type
The following table lists the standard program and safety program execution types.
Safety programs are executed as a fixed scan execution type program. However, safety programs perform safety control, and
therefore operation differs from standard fixed scan execution type programs in the following ways.
• They are unaffected by disabling interrupt instructions (EI instruction, DI instruction, and IMASK instruction), and are
executed based on the safety cycle time.
• Of all interrupt programs that can be created by customers, the execution of safety programs is given highest priority.
• Safety cycle processing including safety programs prioritizes fixed periodicity. If the safety cycle time is exceeded, and
safety cycle processing is executed, processing is not performed in succession, but based on the interrupt timing of the
next cycle time.
When the program does not complete within the safety cycle time
Whether execution of the safety program is completed within the safety cycle time is monitored, and if the program execution
time exceeds the next safety cycle time, an error occurs at the SIL2 Process CPU and SIL2 function module. Note that if the
next safety cycle time is exceeded, the safety input safety program is not executed following program completion, a safety
cycle processing error (error code: 1A01H) is detected at the SIL2 Process CPU, and the safety program is executed at the
next safety cycle time. Also, if execution of the program is completed after the safety cycle time has been exceeded, the
safety cycle processing time execution cycle error flag turns on, and the safety cycle processing time execution cycle error
count is incremented by one as follows.
Program Execution type
Standard program • Initial
• Scan
• Fixed scan
•Event
• Standby
Safety program Fixed scan
Item Safety cycle processing time
execution cycle error flag
Safety cycle processing time
execution cycle error count
Cause of increments of safety cycle
processing time execution cycle error
count
SIL2 Process CPU SM1888 SD1888 Occurrence of an error code: 1A00H or 1A01H
SIL2 function module Un\G54 Occurrence of an error code: 1A00H
Tsc: Safety cycle time
Tsio: Safety program + Safety input/output processing time
(1): If safety input/output and safety program execution are not completed within the safety cycle time, interrupts during this period are ignored. Execute safety
input/output and safety programs from the next interrupt.
(2): If the time from safety input to safety output exceeds the safety cycle time, a continuation error will occur after safety processing is complete.
• SIL2 Process CPU: SM1888 turns ON, and the value in SD1888 is incremented by one.
• SIL2 function module: The value in Un\G54 is incremented by one.
CBACBA
TscTscTsc
Tsio Tsio
(1)
(2)
Safety output
(output refresh)
Safety program
(operation processing)
Safety input
(input refresh)
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