Interrupt Priority - Siemens SIMATIC S5-90U System Manual

Onboard Interrupt Inputs
10.3

Interrupt Priority

In the S5-90U:
You cannot stop interrupt processing once it has started. If an interrupt and a counter overflow
occur simultaneously, interrupt processing has priority.
Only one new cause for an interrupt can be stored while interrupt processing is running. If more
causes for interrupts are present than can be stored, then they are ignored.
In the S5-95U:
Interrupt-driven program execution (OB3) cannot be interrupted by another interrupt. The relevant
program steps are processed sequentially instead.
Up to eight new OB3 calls can be stored while interrupt processing is running. They are processed
in the order of their occurrence ("queue").
For each interrupt that occurs, a separate diagnostic byte with the relevant interrupt information (on
the channel triggering the interrupt) is available.
If several interrupts occur simultaneously or if interrupt processing is disabled by additionally used
PLC functions (see Table 10.6), the corresponding number of entries is made in the diagnostic byte
(several interrupt-triggering channels are specified). However, OB3 is called only once for these
interrupts.
If interrupts and a counter overflow occur at the same time, the following priorities are adhered to
during execution:
1. OB3 call: Counter B
2. OB3 call: Interrupt inputs
3. OB3 call: Counter A
The diagnostics information (IB 35) for the interrupts and counter A are stored in the queue while
counter B is being processed.
Reaction of the PLC to "Interrupt Disable" through STEP 5 Operation "IA":
The PLC recognizes interrupts and stores the relevant diagnostics information in the queue (same
procedure as if OB3 execution already running) while interrupt processing is disabled by the "IA"
operation (see Section 8.2.8).
Reaction of the PLC to "Interrupt Enable" through STEP 5 Operation "RA":
The queue is processed in the case of an interrupt enable through the "RA" operation (see Section
8.2.8); for each dianostics entry in the queue, an OB3 will be started and the cause of the interrupt
flagged in IB 35.
10-4
(I 35.1=1)
(I 35.4 to 35.7=1)
(I 35.0=1)
S5-90U/S5-95U
EWA 4NEB 812 6115-02b