Sonifex RB-OA3C Expansion Unit Cable For RB-OA3If you add another RB OA3 to an existing unit to expand either the number of busses or studios, you need additional cables to connect it which are contained in this kit 1 x Transmission output expansion cable, 25 pin D type male to 25 pin D type male, 30cm lead, wired pin to pin 1 x LSO expansion cable, 9 pin D type male to 9 pin D type male, 30cm lead, wired pin to pin 1 x RS232 expansion cable, RJ45 to RJ45 standard wiring, 30cm lead 1 x kit should