Siemens Simatic M7-400 Reference Manual page 284

Analog Modules
Hardware Interrupt with Trigger "Upper or Lower Limit Exceeded"
Define a working range by setting parameters for an upper and lower limit value. If
the process signal (for example, the temperature) leaves this working range, the
module triggers a hardware interrupt, provided the interrupt is enabled.
The CPU interrupts execution of the user program and processes the hardware
interrupt block (OB 40).
In the user program of OB 40, you can set how the programmable controller is
required to react to a limit value being surpassed or not being reached.
When OB 40 is exited, the hardware interrupt is acknowledged on the module.
Note
Note that a hardware interrupt is not triggered if you have set the upper limit above
the overrange or the lower limit below the underrange.
Structure of the Start Information Tag OB40_POINT_ADDR of OB 40
The limit values exceeded by the different channels are entered in the start
information of OB 40 in the tag OB40_POINT_ADDR. The following figure shows
the assignment to the bits of local data double word 8.
LB 8
31 30 29 28 27 26
25 24
1
Value exceeds upper limit for channel 1
Figure 5-24 Start Information of OB 40: Which Event Triggered the Hardware Interrupt at the Limit Value
5-68
LB 9
.1 .0
17 16
1 1
1
Value falls below lower limit for channel 0
Value falls below lower limit for channel 1
Value exceeds upper limit for channel 0
S7-400, M7-400 Programmable Controllers Module Specifications
LB 11
1 0 Bit no.
LD 8
A5E00069467-07