Allen-Bradley LOGIX 5000 Reference Manual page 285

Drives
Execution
Function Block
Condition Function Block Action
Prescan EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is false EnableIn and EnableOut bits are cleared to false.
Tag.EnableIn is true EnableIn and EnableOut bits are set to true.
The instruction executes.
Instruction first run N/A
Instruction first scan In
Postscan EnableIn and EnableOut bits are cleared to false.
Structured Text
Condition/State Action Taken
Prescan See Prescan in the Function Block table.
Normal Execution See Tag.EnableIn is true in the Function Block table.
Postscan See Postscan in the Function Block table.
Examples
Example 1
The most common use of the PMUL instruction is in the relative mode of
operation. In this mode, the PMUL instruction serves several purposes. First, in
the relative mode, the PMUL instruction differentiates the information that it
receives at its input from scan to scan. As data is received, the instruction outputs
the difference of the input from one scan to the next. This means that if In = 500
at scan "n", and then In = 600 at scan "n+1", Out = 100 at scan "n+1."
Secondly, while in this mode of operation, the PMUL instruction also
compensates for "rollover" values of binary data originating from a feedback
module. For example, a resolver feedback module may have 12 bits of resolution,
represented as a binary value, with sign, ranging from -2048 to 2047. In terms of
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
= In. Out =0. Reminder = 0.
n-1 n-1
Chapter 2
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