Siemens SIMATIC S5-90U System Manual page 286

Onboard Interrupt Inputs
More than eight Causes of Interrupts are Signalled to the S5-95U:
If more causes of interrupts are present than can be stored in the "queue" (> eight interrupts), the
corresponding number of additional entries is made in the diagnostic byte for the eighth interrupt
(specification of several channels triggering interrupts).
If several interrupts in succession are triggered on one channel, the PLC is not able to differentiate
between them (since associated bit is already set).
Example: During interrupt processing, a total of nine additional causes of interrupts are present.
The eighth cause of interrupt is present at I 34.3. The ninth cause of interrupt is present
at I 34.2. Both causes are combined and entered in diagnostic byte IB 35.
The associated diagnostic byte is as follows:
Bit No.: 7 6 5
1 1 ...
IB 35
Bits not relevant for interrupt processing
Figure 10-2. Example of Acquisition of Interrupt Causes in the Diagnostic Byte
If the 9th cause of interrupt was also present at I 34.3, this interrupt would not be noticed.
10-6
4 3 2 1 0
... ... ...
I 34.2; 9th cause of interrupt is flagged
I 34.3; 8th cause of interrupt is flagged
EWA 4NEB 812 6115-02b
S5-90U/S5-95U