Allen-Bradley LOGIX 5000 Reference Manual page 286

Chapter 2 Drives
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raw data coming from the feedback module, the rotation of the feedback device
might be represented as shown below:
In this example, as the value of the feedback data moves from 2047 to -2048, the
effective change in position is equivalent to a jump of 4095 counts in position. In
reality, however, this change in position is only 1 part in 4096 in terms of the
rotation of the resolver feedback device. By understanding the true word size of
the data that is being input from the feedback module, the PMUL instruction
views the data in a rotary fashion as shown in the following diagram:
By knowing the word size of the data that is input to this block, the PMUL
instruction differentiates an output of 1 count as the input to the block moves
from 2047 to -2048, instead of the mathematically calculated 4095.
Tip: When applying this block, it is important to note that the feedback data should not change by more than one-
half of the word size from one scan to the next, if rotational direction is to be properly differentiated.
In the example above, if the feedback device is moving in a clockwise direction
such that at scan 'A' it reads 0 and then scan 'B' it reads -2000, actual change in
position is equivalent to +2096 counts in the clockwise direction. However, since
these two values are more than one half the words size, (or more than one half the
rotation of the physical device,) the PMUL instruction calculates that the
feedback device rotated in the opposite direction and returns a value of -2000
instead of +2096.
The third attribute of the pulse multiplier block is that it retains the fractional
components from one scan to the next of any remainders that exist as a result of
the Multiplier/100,000 scaling factor. As each execution of the block is completed,
the remainder from the previous scan is added back into the total of the current
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018