Amazon.com: APC AP940-1525A UPS Communications Smart-UPS Signalling Serial RS-232 Cable DB9 to RJ45 (RJ50) - Same as AP940-1525C and AP940-0625A But a Longer Cable : Electronics
APC Genuine UPS SMX SMT Series Serial Cable RS-232 to RJ-50 940-0625A
APC RJ12 serial cable pinout diagram @ pinoutguide.com