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IP 266 STEP 5 Programming

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 Cycle n+1 Cycle n+2 Cycle n+3 Cycle n+4

Mode is

started

Response

when mode

is started in

cycle n-1 or

earlier, i.e.

status info

is not up to

date

Earliest

response to

invocation

of a mode

in cycle n

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"out of date" information.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

9-1

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Siemens IP-Module Manual Pagina 289