Siemens SIMATIC S5 Manual page 84

Introduction to STEP 5
• User interface:
When hardware interrupt A occurs, the operating system processes OB 2 after a maximum of
30 msec.
If the interrupt OB is not programmed, cyclic program scanning continues.
• Interrupt locations:
After a maximum of 20 msec., a program is called in the operating system to see if an interrupt
OB must be processed. If an interrupt is pending in both subunits, the program branches into
OB 2.
If the 20 msec. interrupt processing intervals (30 msec. max. interrupt response time) are also
to be observed in the user program, an FB 254 must be called in the user program after a
maximum of 20 msec. This FB then synchronizes interrupt processing in both subunits by
assigning the relevant parameters ( 6.1.6).
• Disable interrupt:
The IA operation disables interrupt handling (OB 2, OB 13); RA enables it. The default setting
is RA. An interrupt can be stored during interrupt disable.
If interrupt handling is not required, the user can reduce PLC scan time.
Disable interrupt handling with the IA operation and switch off interval monitoring when
initializing the operating system with COM 115F.
• Nesting depth:
Even with interrupt handling, the general block nesting depth of 16 levels must not be ex-
ceeded.
• Saving data:
If the interrupt block uses scratch flags that are also used in the cyclic or time-driven control
program, these flags must be saved in a data block while the interrupt is being serviced.
• Synchronization of process interrupt handling
There is a considerable difference between implementation of interrupt handling in the
S5-115F and in the S5-115U. This is due to the synchronization of the two interrupts in subunits
A and B.
Possible methods of interrupt handling:
- Operating system:
- Control program:
The operating system monitors the call interval in the control program. The user configures
the desired maximum time in the operating system with COM 115F.
Actual interrupt processing takes place in OB 2. The user uses the LPB operation exclusively to
read in the interrupt register. In doing so, the contents of the interrupt register are exchanged
between both subunits and compared. Interrupt bits set in both subunits are processed
immediately, and interrupt bits set in only one subunit are stored until the next interrupt is
serviced, but they must be set at the latest after expiry of the interrupt discrepancy time in the
other subunit. Otherwise the S5-115F will stop and set an error flag.
2-10
every 20 msec.
by calling an FB 254 SYNC ( 6.1.6)
S5-115F Manual
EWA 4NEB 811 6149-02