The APC Back-UPS Pro uses a proprietary DB9 serial interface cable (APC part number 940-0095, typically supplied as revision 940-0095B or 940-0095A) to communicate with a host computer running APC PowerChute software. The cable carries both simple (handshaking-line) signalling and the full APC Smart serial protocol at 2400 bps 8N1, making it a hybrid PnP design that allows software to select the appropriate mode. The Back-UPS Pro implements the Basic Port defined in APC Application Note #12, which includes a remote on/off input on pin 7 but does not supply the +24 Vdc output found on larger Smart-UPS models.
9 pin D-SUB male connector to the UPS
9 pin D-SUB female connector to the computer
| 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=4kOhm to pin 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.
Loopback wiring on the computer connector (R = 4 kΩ):
| Pin | Via | Pin |
|---|---|---|
| 1 | 4 kΩ resistor | 3 |
| 3 | 4 kΩ resistor | 1 |
| 2 | shorted | 9 |
| 4, 9 | joined | 5 |
| 4, 8, 6 | shorted | — |
Another variant:
| Computer (DB9F) | Via | UPS (DB9M) | Function |
|---|---|---|---|
| DTR 4, CTS 8, DSR 6, DCD 1 | shorted together | handshake loop | |
| GND 5 | direct | 4 | Ground |
| GND 5 | direct | 9 | Common |
| RI 9, RxD 2 | joined | 2 | On Battery |
| TxD 3 | 4.7 kΩ resistor | 1 | Kill UPS power |
Notes
The UPS-end connector is a DB9 male; the computer-end connector is a DB9 female. The cable is electrically asymmetric — do not reverse the connectors.
The 940-0095 is a hybrid PnP cable: it contains the handshaking-line wiring of the simple-signalling 940-0020 and the serial data lines of the smart-signalling 940-0024. PowerChute uses the jumpered pins on the cable to identify the cable type automatically.
APC Application Note #12 states that the Back-UPS Pro accepts a HIGH RS-232 level on pin 1 (Shutdown) for 6 seconds to initiate a controlled shutdown, and that this shutdown only takes effect whilst the UPS is running on battery. The pin 7 remote on/off feature is available on the Back-UPS Pro but no +24 Vdc supply is present on pin 8 (that supply is reserved for Smart-UPS and Matrix-UPS models only).
The APC smart serial protocol operates at 2400 bps, 8 data bits, no parity, 1 stop bit (8N1). Note the non-standard pin assignment on the UPS side: TxD is on UPS pin 1 and RxD is on UPS pin 2, the reverse of standard RS-232. Connecting a standard straight-through serial cable risks driving the UPS shutdown line immediately.
Suffix letters in part numbers (e.g. 940-0095A vs 940-0095B) indicate minor physical revisions only; the wiring is electrically identical across suffix variants.
Signals
- RX / Shutdown (UPS pin 1) — shutdown input; UPS shuts down after receiving a HIGH RS-232 level for 6 seconds (Back-UPS Pro); also carries TxD in smart serial mode (non-standard: TxD on pin 1, not pin 3)
- TX / AC Fail (UPS pin 2) — on-battery output; transitions from LO to HI RS-232 level when the UPS is running on battery; also carries RxD in smart serial mode at 2400 bps 8N1
- Common (UPS pins 4, 9) — chassis-referenced signal common; connected to computer GND (pin 5)