The BK Precision DRRS-485 RS-485 Interface Card is designed for use with the 9170 or 9180 Programmable Dual-Range DC Power Supplies (not included). This card will upgrade the main unit so that RS-485 interface is possible. With the DRRS-485 installed, multi-unit control mode for the main unit is enabled. While in this mode, up to 31 units can be daisy-chained via the RS-485 interface for synchronization purposes. This mode allows for these units to be controlled from one master unit via the USB, GPIB, or LAN interface.