In brownfield PLC retrofits, analog output scaling mismatches between Allen-Bradley ControlLogix or Honeywell HC900 controllers and legacy field devices cause hidden control errors. This guide covers the most common AO scaling failure patterns: raw count range mismatches, engineering unit offset errors, and clamp limit interactions. Step-by-step correction procedures and commissioning test methods help engineers verify correct 4–20 mA delivery before process restart.
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