Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 170)

Block Transfer
Chapter 14
14-15
When two or more block transfer instructions have a common module
address, program carefully to compensate for the following
possible situations:
First - During any program scan, data in the output image table byte can
be changed by each successive block transfer instruction having a common
module address. The enable bit can be turned on or off according to the
true or false condition of the rungs containing these instructions. The on or
off status of the last rung governs whether the transfer occurs.
Second - The block length can be changed according to the block lengths
of the enabled instructions. The block length of the last enabled block
transfer instruction having a common module address governs the number
of words transferred.
ATTENTION: When programming multiple writes (or reads)
to the same module, it is possible that a desired transfer does not
take place or the number of words transferred is not the number
intended. Invalid data can be sent to a device (or can be
operated upon in subsequent scans) resulting in unpredictable
and/or hazardous machine operation.
See the module user’s manual for any information unique to that module.
The example in Figure 14.7 shows how multiple reads of different block
lengths from one module can be programmed. When any one of the input
switches is closed, the rung is enabled and the lock length is established.
The last rung enables the block transfer instruction regardless of the
previous changes in status of the enable bit. The Examine Off instructions
prevent more than one of the block transfer instructions from being
energized in the same scan.
Important: The same discussion applies when programming multiple
writes of different block lengths to one module.