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Chapter 5
Offset Programming
Programming Considerations
with Offset
5 6
The default block lengths (00) for block-transfer instructions are 20
block-transfer-write words and two block-transfer-read words. These are
the block lengths that transfer to and from the absolute encoder regardless
of whether you use the offset feature.
When you have an offset value and the module is controlling eight
outputs, for example, the number of words you send to the module is 22.
You must enter the numbers 22 and 2 for the block lengths of write and
read data. Do not enter the default block length in your instructions if you
use the module's offset feature.
For PLC-2 family processors, do not enable the read- and
write-block-transfers in the same scan when you use the offset feature.
An example program enabling the instructions in separate scans follows.
WARNING: When the block lengths of bidirectional
block-transfer instructions are set to unequal values, do not
enable the rung containing the alternate instruction until the
done bit of the first transfer is set. If you enable them in the
same scan:
the number of words transferred may not be the number
intended
invalid data could be operated upon in subsequent scans
output devices could be controlled by invalid data
Unexpected and or hazardous machine operation could occur.
Damage to equipment and/or injury could result.
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