9 pin D-SUB female connector to the computer
10 pin RJ50 (10P10C) male connector to the UPS
APC Back-UPS CS and ES models with a serial port use a proprietary RJ45 or RJ50 (10P10C) interface at the UPS end, paired with a standard DB9 female connector at the computer end. The official 940-0128A (and 940-0128D) cable provides simple signalling only, but the same physical connector can carry smart signalling — the full bidirectional ASCII serial protocol running at 2400 bps, 8 data bits, no parity, 1 stop bit — by cross-connecting the serial data lines as shown below. APC smart serial ports use a non-standard pin assignment in which TxD appears on pin 1 and RxD on pin 2 at the UPS end, the reverse of normal RS-232. These UPS models provide a Basic Port only, with no Pin 7 remote on/off and no Pin 8 supply voltage.
| PC Signal | PC pin | UPS Pin | UPS signal |
|---|---|---|---|
| RxD | 2 | 2 | TxD |
| TxD | 3 | 8 | RxD |
| GND | 5 | 7 | Ground |
| FG | Shield | 4 | Frame Ground |
APC supplies a serial cable (typically with a green DB9 F connector) stamped with part number 940-0128A. Below is the wiring scheme of the original 940-0128A cable.
APC part 940-0128A — DB-9F (computer) to RJ45 (UPS).
| Computer (DB9-F) | Via | Signal | UPS (RJ45) |
|---|---|---|---|
| DSR 4, DTR 6 | diode + resistor | Kill power | 8 Orange |
| DCD 1, RxD 2 | direct | Low battery | 3 Brown |
| RTS 7 | two resistors | status drive | |
| RI 8, CTS 9 | direct | On battery | 2 Black |
| GND 5 | direct | Signal ground | 7 Red |
| Chassis / GND | direct | Chassis ground | 4 Black |
| — | — | not connected | 1, 5, 6, 9, 10 |
Notes
The UPS-end connector is a proprietary RJ45 or RJ50 (10P10C) modular plug; despite the physical similarity to standard Ethernet RJ45, it is not electrically compatible. At the computer end, the DB9 female connector carries RS-232 levels. Smart signalling uses 2400 bps, 8N1 framing with software flow control; only three conductors (TxD, RxD, GND) are required for the serial protocol. The APC smart port maps TxD to UPS pin 1 and RxD to UPS pin 2 — the reverse of standard RS-232 — so a straight-through cable will not work and may cause unintended UPS behaviour. The 940-0128A and 940-0128D cables support simple (dumb) signalling only; the wiring in the table above is an alternative arrangement for smart signalling on the same physical connector type.
Signals
- RxD — Receive data at the computer (serial data from UPS TxD)
- TxD — Transmit data from the computer (serial data to UPS RxD)
- GND — Signal ground reference
- FG — Frame/chassis ground (cable shield)