The ABB CI856K01 with part number 3BSE026055R1 is a rack-mounted dedicated bus interface module for ABB AC 800M controllers under System 800xA DCS, designed exclusively for bridging modern AC 800M CPU and legacy S100 distributed I/O stations. Unlike serial-based CI853K01, it adopts a dedicated 36-pin Miniribbon rear connector for high-speed parallel I/O bus transmission, realizing synchronous data interaction between up to 8 S100 I/O racks in a single bus loop. It draws 24V DC backplane power from the AC800M CEX bus, with ultra-low typical operating current of 120mA and only 2.88W rated power consumption, featuring built-in reverse polarity protection and transient surge suppression to cope with industrial power ripple. It provides 500V DC galvanic isolation between the CEX backplane and S100 field bus, eliminating ground loop interference caused by potential differences across scattered field cabinets. Physical parameters are standardized for AC800M rack installation: dimensions 185mm × 59mm × 127.5mm, net weight 0.7kg, occupying two standard rack slot positions. Three front-panel LED indicators independently show module power status, CEX bus link and S100 bus communication activity, enabling offline fault diagnosis without Control Builder M software. It complies with IEC 61000-6-2 industrial EMC standards, with operating temperature ranging from 0°C to 55°C, storage temperature from -40°C to 70°C, and 5% to 95% non-condensing humidity tolerance under IP20 enclosure rating. It supports verified hot swapping when controller CPU load is below 40% without disrupting running control logic. Manufactured in Sweden with CE, RoHS and UL certifications, it is a core retrofit spare part for upgrading outdated S100 I/O systems instead of full cabinet replacement, widely deployed in metallurgy, power plant and chemical legacy DCS renovation projects.