APC rack-mount Smart-UPS models (SU, SUA, SUM, and SURT platform prefixes) communicate with a host computer over a proprietary DB9 serial interface using the smart signalling protocol. The 940-1524C cable is a 5-metre variant of the standard 940-0024C smart signalling cable and carries the same wiring; the serial protocol operates at 2400 bps, 8 data bits, no parity, one stop bit. The UPS end carries a DB9 male connector and the computer end a DB9 female. APC’s advanced port on these models also provides a remote on/off line (pin 7) and an unregulated +24 Vdc supply (pin 8), though these are not wired through the 940-0024/1524 smart cable. Newer rack platforms (SMT, SMX, SURTD, SRT prefixes) switched to a keyed RJ45 port and require a different cable family entirely.
9 pin D-SUB male connector
9 pin D-SUB female connector
| UPS Name | UPS Pin | Direction | PC Pin | PC Name | Description |
|---|---|---|---|---|---|
| RX | 1 | 3 | TX | ||
| TX | 2 | 2 | RX | ||
| C | 9 | 5 | G |
Notes
- Pins 7 and 8 are shorted on PC side. Pins 1 and 4 are shorted on PC side.
- The pin assignment is non-standard: TxD appears on UPS pin 1 and RxD on UPS pin 2, the reverse of standard RS-232. Connecting a straight-through serial cable to an APC smart UPS risks driving the UPS on/off line and shutting down the connected computer.
- Part number suffix letters (e.g. 940-1524C vs 940-1524A) indicate only minor physical or OEM revisions; the wiring is electrically identical across all suffix variants.
- The 940-1524C is the 5-metre length variant; the 940-0024C is the standard 6-foot equivalent. Both carry identical smart signalling wiring.
Signals
- RX (UPS pin 1) — UPS receive data; wired to PC pin 3 (TxD). Non-standard placement: this is the UPS data-receive line despite occupying the pin position conventionally associated with TxD.
- TX (UPS pin 2) — UPS transmit data; wired to PC pin 2 (RxD). Carries the APC smart protocol at 2400 bps 8N1.
- C (UPS pin 9) — Chassis ground; wired to PC pin 5 (signal ground).