Siemens S5-155U System Manual page 311

Scanning the process inputs
Settings on the module
System Manual
C79000-B8576-C199-03
So that the module can initiate a process alarm for a change of logic
state, the alarm must be processed by an alarm routine (alarm OB) in
the user program, with automatic acknowledgement of the alarm. To
avoid loss of an alarm, it must not be possible for the alarm OB to be
interrupted by process alarms. The CPU must process the process
alarms with level triggering (see Programming Guide of the CPU).
Example:
Program the following in the alarm OB which you have preset with
byte IB 0 and switch S2:
L PW128
T FW0
L PW130
T FW2
Only the flag word (FW) may be accessed during the cyclic program.
Accessing the I/O bytes in the process image will result in alarm loss.
Addressing of the module must therefore be above address 127.
Double accessing of I/O bytes, even from different CPUs, is not
permissible (alarm loss).
The four I/O bytes of a module must be scanned successively and in
ascending order. The scanning of byte n inhibits the input circuit of
the module, and the scanning of byte (n + 3) enables them again.
Make the following settings on the module to operate it with a process
alarm via IB 0:
Step Action
1 Insert jumpers X3 and X4.
2 Set switch row S1 to Off.
3 Set bit 0 on switch row S2 to On, and all other switches to
Off.
4 Set switch row S3 to choose whether the interrupt is to be
initiated with a positive-going (leading) or negative-going
(trailing) edge. The setting of a pair of switches applies to
an entire byte.
Technical Description
(load I/O word)
(transfer flag word)
etc.
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