Rockwell Automation 1785 PLC-5 User Manual download pdf (Page 95)

Switchover Considerations

Chapter 6

6-10

Because the secondary processor’s program scan and block transfer

execution are totally asynchronous, it is possible for the secondary

processor to operate on partial data structures. If the secondary processor

is executing instructions that operate on any of the multi-word structures

that are listed above, the following sequence of events may occur:

1. The 1785-BCM transfers the first word of a multi-word structure to

the secondary processor. A partial data structure temporarily exists in

the secondary processor.

2. The secondary processor operates on this partial structure and derives

invalid results.

3. The partial data structure and the invalid result of the operation exist

in the secondary processor until the remaining portion of the structure

is transferred. The secondary processor re-executes the operation on

the next program scan and overwrites the invalid result with the

correct result.

4. If the primary processor were to go de-active before the invalid result

of the processor operation was corrected, then the invalid results

would persist in the secondary processor for an indeterminate period

of time.

Important: To prevent partial structures from occurring in the secondary

processor, you must group related data words or multi-word elements (i.e.

floating point) so that they fit in one block.

Some of the PLC-5 processor instructions operate on data structures with

less than 16-bit words. These other data structures are:

bits

bytes

bit fields

Instructions such as bit-shift and bit-distributor instructions use these other

data structures. When using these instructions, you must be aware that the

data table will contain invalid data until the processor executes a series of

instructions. For example, if you are shifting four bits of BCD data

through several words bit by bit, the data table will contain invalid data

until each shift is complete.

Data Integrity

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Rockwell Automation 1785 PLC-5 User Manual Page 95