connector or cable wiring APC Part# - 940-0023A
9 pin D-SUB female connector to the computer
The APC 940-0023A is a simple-signalling (dumb) serial cable for Back-UPS models that carries only the On Battery status signal using RS-232 handshaking lines, with no serial protocol involved. It belongs to the APC 940-series proprietary interface cable family, which was introduced alongside APC’s Back-UPS line in the early 1990s. The cable connects a DB9 male port on the UPS to a DB9 female port on the computer. Because it conveys only the On Battery condition and omits the Battery Low and shutdown signals provided by the fuller 940-0020 simple-signalling cable, its practical utility is limited.
| Computer (DB9F) | Via | UPS (DB9M) | Function |
|---|---|---|---|
| DCD 1 | direct | 2 | On Battery |
| GND 5 | direct | 4 | Ground |
| GND 5 | direct | 9 | Common |
On the computer side, TxD (3) is tied to DTR (4) through a 3.3 kΩ resistor to hold DTR asserted.
9 pin D-SUB male connector to the UPS
Notes
- The UPS DB9M port uses APC’s Basic Port (simple/dumb) signalling; no serial protocol is spoken on this cable.
- UPS pin 2 (On Battery) is an open-collector output that transitions from a LO to a HI RS-232 voltage level when the UPS switches to battery power; the computer receives this on pin 1 (DCD).
- UPS pin 4 is the signal common; UPS pin 9 is the chassis ground. Both are tied to computer pin 5 (GND) via a shared junction.
- A 3.3 kΩ resistor is placed between computer TxD (pin 3) and the DTR (pin 4) junction; this biases the line but does not connect to any UPS input — no shutdown command is wired in this cable variant.
- The cable carries no Battery Low signal and no shutdown input, distinguishing it from the 940-0020 simple-signalling cable, which includes those additional lines.
- The APC computer interface port is proprietary; APC intends it for use only with APC software, cables, and accessories.
Signals
Computer side (DB9F)
- DCD (pin 1) — On Battery input: goes HI when the UPS is running on battery
- TxD (pin 3) — tied through 3.3 kΩ to DTR; biases the line, no UPS connection
- DTR (pin 4) — joined to TxD via resistor; no UPS signal connected
- GND (pin 5) — signal ground; connected to UPS pins 4 and 9
UPS side (DB9M)
- On Battery (pin 2) — open-collector output; LO→HI RS-232 transition when UPS is on battery
- Ground (pin 4) — signal common for open-collector outputs
- Common (pin 9) — chassis ground reference