APC BackUPS Pro PNP cable

     UPS end                                      Computer end
     -------                                      ------------
                       47k        47k
     BATTERY-LOW (5) >----R1----*----R2----*----< DTR,DSR,CTS (4,6,8)
                              |          |
                              |          |
                              |         /  E
                              |       |/
                              |    B  |
                              *-------|  2N3906 PNP
                                      |
                                      |\
                                        \  C
                                         |
                                         |
                                         *----< DCD (1)     Low Batt
                                         |
                                         |
                                         R 4.7k
                                         3
                                         |
                                  4.7k   |
     SHUTDOWN (1)    >----------*----R4----*----< TxD (3)
                              |
                              |  1N4148
                              *----K|---------< RTS (7)      Shutdown
     
     POWER-FAIL (2)  >--------------------------< RxD,RI (2,9) On Batt
     
     GROUND (4,9)    >--------------------------< GND (5)

DTR is "cable power" and must be held at SPACE. DSR or CTS may be used as a loopback input to determine if the cable is plugged in. DCD is the "battery low" signal to the computer. A SPACE on this line means the battery is low. This is signalled by BATTERY-LOW being pulled down (it is probably open circuit normally).

TxD must be held at MARK; this is the default state when no data is being transmitted. This sets the default bias for both DCD and SHUTDOWN. If this line is an open circuit, then when BATTERY-LOW is signalled, SHUTDOWN will be automatically signalled; this would be true if the cable were plugged in to the UPS and not the computer, or if the computer were turned off.

RTS is the "shutdown" signal from the computer. A SPACE on this line tells the UPS to shut down.

RxD and RI are both the "power-fail" signals to the computer. A MARK on this line means the power has failed.

SPACE is a positive voltage, typically +12V. MARK is a negative voltage, typically -12V. Linux appears to translate SPACE to a 1 and MARK to a 0.