Siemens SIMATIC S5 Manual page 258

Application of the Function Block
Reading the image of the digital outputs without FB 162:
The status indication of the image of the digital outputs is normally updated by reading
message 19.
The function block FB 162 must have the command IW (read actual value) and
message number 19 assigned.
To update the image quickly, it is possible to read only these three bytes of the image. To do this,
certain requirements must be fulfilled.
The following diagrams describe ways of updating the image of the digital outputs (DB-B, DW 94
and DL 95) for specific PLCs.
S5-115U:
FB x = UPD.DQI
S5-135U CPU 922 and CPU 928 when interrupts are possible at the block boundaries and
S5-155U in the 150U mode.
FB x = UPD.DQ1
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IP244
C79000–B8576–C861–02