Siemens SIMATIC S5 Manual page 290

STEP 5 Programming
9.1.2 Edge Evaluation and Modes
The preferable technique to ensure a readout of the latest status info is edge
evaluation.
Sequence:
1. A job is started in cycle n. The status info shows "Job terminated".
2. The status info still shows "Job terminated" in cycle n+1 because the job had
not yet been started when the process input image was read.
Note:
If this status bit were the only criterion for starting a new job, it would be
possible to start another job at this point although the first is still in progress.
The first job would thus be aborted and error F82 ("Inadmissible job") flagged.
The IP 266 reports "Job in progress" in cycle n+2 at the earliest.
It would therefore be preferable to use an edge flag to make sure that this pro-
blem cannot occur. No job can be started unless this flag is set, and it is set only
when the IP 266 returns "Job in progress".
Application in example 2 (see Section 9.5):
When a job is started, flag 3.0 is reset. As long as this flag is not set, no job can be
started. The flag is not set and a further job initiated until the IP 266 reports "Job
in progress".
A new job can be started when the IP 266 changes the status bit from "Job in
progress" to "Job terminated".
9-2
IP 266
EWA 4NEB 812 6057-02