SERIAL INTERFACE UNIT (SIU). THIS IS AN ELECTRICAL COMPONENT THAT FORMS PART OF AN ELECTRICAL CONTROL SYSTEM. THE FUNCTION OF THE SIU IS TO INTERFACE BETWEEN THE CONTROL PANEL (PLC) AND EXTERNAL DEVICES SUCH AS VALVES, PNEUMATIC DEVICES, SENSORS, CARRYING OUT INSTRUCTIONS SUCH AS SWITCHING ON/OFF. THE SIU CONTAINS COMPONENTS SUCH AS MICROPROCESSORS, MEMORIES, POWER REGULATORS, OSCILLATORS, ACTUATORS, COMMON PASSIVE COMPONENTS MOUNTED ON PRINTED CIRCUIT BOARDS. THE CONTROL PANEL CANNOT OPERATE WITHOUT THE SIU.

GENERAL RULES 1 AND 6 FOR THE INTERPRETATION OF THE COMBINED NOMENCLATURE. CHAPTER NOTE 9(IV) TO CHAPTER 85. THE HS EXPLANATORY NOTES TO HEADING 8538. THE TEXT TO CN CODES 8538 90 99.