9 pin D-SUB female connector to the computer
10 pin RJ50 (10P10C) male connector to the UPS
The APC part number 940-0128D is the simple signalling cable supplied with Back-UPS XS1000, BX-1000 and related models. It connects a 9-pin D-SUB female connector at the computer end to a 10-pin RJ50 (10P10C) modular plug at the UPS end and carries only RS-232 handshaking-line signals — no serial protocol is used. The cable belongs to the APC Basic Port family, which uses open-collector outputs and discrete voltage-level transitions to communicate on-battery and low-battery states, and accepts a high RS-232 level on the shutdown input to switch off the UPS after approximately one second.
940-0128D is functionally similar to the 940-0128A cable except for NC on (6) DTR and (2) RD on the computer side.
| Computer (DB9) | Via | UPS | Function |
|---|---|---|---|
| DTR (4) | direct | 8 | Kill power |
| GND (5) | direct | 7 | Signal ground |
| Shield | direct | 4 | Chassis ground |
The On-Battery and Low-Battery status lines are produced by a small transistor circuit: RTS (7) drives two 2N2222 NPN transistors through 2 kΩ resistors, whose collectors pull CTS/RI (8,9) [On Battery] and DCD (1) [Low Battery] via 1N5819 Schottky diodes.
Notes
The DB9 connector at the computer end is female (DE-9F); the RJ50 (10P10C) modular plug mates with the UPS’s proprietary keyed port. Pin 4 (COMMON) at the UPS end serves as the reference common for the open-collector output signals; pin 7 (Signal GND) connects to pin 5 at the computer. The transistor circuit (2N2222 NPN with 2 kΩ resistors and 1N5819 Schottky diodes) level-shifts the on-battery and low-battery signals from the UPS onto the RS-232 handshaking lines. Pin 8 (KillPwr) at the UPS corresponds to the Basic Port shutdown input: the UPS shuts down approximately one second after receiving a sustained HIGH RS-232 level. Pins 6 (DTR at computer) and 2 (RD at computer) are not connected (NC) on the 940-0128D, distinguishing it from the 940-0128A variant.