Siemens SIMATIC S5 Manual page 289

IP 266
9 STEP 5 Programming
9.1 What to Observe when Programming
9.1.1 General Information on Program Structure
When programming the IP 266, you must observe a number of special features
and exceptions. These are discussed in detail below.
Response to starting a mode
When you start a mode, you can expect the latest status info at the earliest two
cycles later. If you are using a high-speed CPU in conjunction with a short OB 1
cycle time, you may not receive this information until the third or fourth cycle.
Cycle n-1 Cycle n
Mode is
started
Process output image is forwarded to IP 266 (readied in cycle n)
Process input image is read from the IP 266 (in cycle n-1)
A special programming technique should be used to prevent repeated reading of
"out of date" information.
Two optional techniques are discussed below:
• A blank cycle can be programmed in each second OB 1 cycle to ensure that
the status info will be read no earlier than cycle n+2. However, this alone
does not ensure that the current status info will be read.
• A preferable technique would be to evaluate the signal edge of the "Job
terminated/in progress" bit.
EWA 4NEB 812 6057-02
Cycle n+1 Cycle n+2 Cycle n+3
Response Earliest
when mode response to
is started in invocation
cycle n-1 or of a mode
earlier, i.e. in cycle n
status info
is not up to
date
STEP 5 Programming
Cycle n+4
9-1