Siemens SIMATIC S5 Manual page 314

STEP 5 Programming
Every "Command FB" includes the following code-evaluation routine:
Input set
yes
AND auxiliary flag
not set?
no
Edge flag = 0
yes
Input set?
no
Auxiliary flag = 0
This routine works with auxiliary flags and edge flags, and ensures that a
command is entered in the output byte only once in the first cycle that follows
setting of the input. Edge flag 4.1 is used for this purpose.
0th cycle Switch/key is actuated
1st cycle Input appears in the PII as being set
Auxiliary flag
Edge flag
2nd to nth cycle Input remains set
Auxiliary flag
Edge flag
9-26
Setting auxiliary flags and edge flags
Edge flags
Edge flag = 1
Auxiliary flags
Auxiliary flag = 1
= 1
= 1
= 1
= 0
IP 266
A I 3.1
AN F 4.0
= F 4.1
A I 3.1
= F 4.0
Frame
is entered in output byte
is not entered in output byte
EWA 4NEB 812 6057-02