PLC D Registers and MOV: Why Scan Order Changes Results

Numeric data in a PLC has execution-order behavior too. Where MOV sits in the scan can change the result.

D registers store numeric data

Bit devices represent ON/OFF state. D registers hold values such as setpoints, measurements and calculation results.

MOV K100 D0

Turning the rung OFF does not automatically clear D0.

Instruction order changes same-scan behavior

A MOV placed above a comparison changes the value seen by that comparison in the same scan. If the comparison is above the MOV, it sees the old value until the next scan.

// A
MOV K110 D0
[D0 >= K100] -> Y0

// B
[D0 >= K100] -> Y0
MOV K110 D0

Multiple writes to the same register

If several MOV instructions write the same D register in one scan, the later write wins. That makes rung order part of the Auto/Manual/Stop priority design.

Y0 AND X5  -> MOV D100 D120
Y0 AND /X5 -> MOV D110 D120
/Y0        -> MOV K0 D120

HMI numeric input

An HMI numeric field can write directly to a D register. A 37.5% command can be stored as D110=375 when the project uses an integer 0–1000 scale.