Siemens SIMATIC S7-400 Reference Manual page 94

Digital modules
4.5 Interrupts of the digital modules
Pending hardware interrupts trigger hardware interrupt processing in the CPU (OB 40 to OB
47). The CPU interrupts the execution of the user program or the priority classes with low
priority.
In the user program of the hardware interrupt OB (OB 40 to OB 47) you can specify how the
programmable controller is to respond to an edge change. The module acknowledges the
hardware interrupt when the program exits the hardware interrupt OB.
For each channel the digital input module can buffer an interrupt that has not been triggered.
If no run levels of a higher priority class are pending processing, the CPU processes the
buffered interrupts (of all modules) in the order of their occurrence.
Hardware interrupt lost
A "Hardware interrupt lost" diagnostics interrupt is generated, if a successive interrupt occurs
at the same channel before the CPU has processed the previously buffered interrupt.
The CPU does not register any further interrupts at this channel unless it has completed
processing of the stacked interrupts of the same channel.
Interrupt-triggering channels
The interrupt-triggering channels are stored in the local data of the hardware interrupt OBs (in
the start information of the corresponding OB). The start information is two words long (bits 0
to 31). The bit number is the channel number. Bits 16 to 31 are not assigned.
See also
General information about parameter assignment (Page 199)
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S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08