9 pin D-SUB male connector to the UPS
9 pin D-SUB female connector to the PC
The APC Back-UPS Office communicates with a host computer through a proprietary simple signalling (Basic Port) interface using APC part number 940-0119A — a DB9 male to DB9 female serial cable. No bidirectional serial protocol is used; the UPS asserts discrete RS-232 voltage levels on handshaking lines to indicate on-battery and low-battery conditions, and accepts a high level on a designated pin to initiate a controlled shutdown. The same Basic Port design is shared across the Back-UPS product family and supports only these hardwired signals; no Pin 7 remote on/off or Pin 8 supply voltage is available on Back-UPS models.
| Pin | Signal | Description |
|---|---|---|
| 1 | 4 and 6 | DSR DTR, Shutdown when set by computer for 1-5 seconds |
| 2 | 8 and 9 | RI CTS, On battery power |
| 3 | 1 and 2 | CD RxD, Low battery |
| 4 | 5 | Ground |
| 5 | 7 | RTS, Begin signalling on other pins |
| 6 |
Notes
The UPS connector is a 9-pin D-Sub male (DB9M); the PC connector is a 9-pin D-Sub female (DB9F). This cable uses simple (dumb) signalling only — no RS-232 serial protocol is involved. The shutdown input responds to a high RS-232 level held for approximately 1–5 seconds. APC describes this port as proprietary and intended solely for use with APC software and accessories.
Signals
- DSR/DTR (PC pins 6/4) — shutdown input: a high RS-232 level held for 1–5 seconds initiates UPS shutdown
- RI/CTS (PC pins 9/8) — on-battery output: asserted by the UPS when operating on battery power
- CD/RxD (PC pins 1/2) — low-battery output: asserted by the UPS when the battery is at a low level
- GND (PC pin 5) — signal ground common
- RTS (PC pin 7) — enables signalling on the other output pins