APC Back Pro UPS cable

9 pin D-SUB male connector to the UPS

UPS Signal Name	DB-9 Male (UPS end)	DB-9 Female (PC end)	PC Signal Name
RX / Shutdown	1	via R=4k to 3	TxD, also connected via R=4k to 1 (DCD)
TX / AC Fail	2	2,9	RxD, RI
Common	4,9	5	GND

At the PC end pins 4,6,8 (DTR, CTS, DSR) shorted.

9 pin D-SUB female connector to the computer