9 pin D-SUB male connector to the UPS
9 pin D-SUB female connector to the computer
The APC 940-0020 is the standard grey simple signalling (dumb) serial cable connecting an APC Back-UPS or compatible UPS to a computer’s DB9 serial port. It uses only RS-232 handshaking lines — no serial data protocol — relying on discrete voltage-level transitions to convey on-battery and low-battery states and to accept a shutdown command from the host. The cable was the primary interface accessory for APC Back-UPS models throughout the 1990s and 2000s and is identified as the DB9-to-DB9 simple signalling cable for Type A UPS in APC’s own documentation. Minor physical revisions are designated by suffix letter (B, C, etc.); all revisions are electrically interchangeable.
| UPS Signal Name | DB-9 Male (UPS end) | DB-9 Female (PC end) | PC Signal Name |
|---|---|---|---|
| Shutdown | 1 | 4 | DTR |
| AC Fail | 2 | 8 | CTS |
| Common | 4,9 | 5 | GND |
APC part 940-0020B
| Computer (DB9F) | Via | UPS (DB9M) | Function |
|---|---|---|---|
| CTS 8 | direct | 2 | On Battery |
| DTR 4 | direct | 1 | Kill power |
| GND 5 | direct | 4 | Ground |
| GND 5 | direct | 9 | Common |
| DCD 1, RTS 7 | semiconductor | 5 | Low Battery |
Note: sides swapped. Full scheme of 940-0020C isn’t yet known.
APC part 940-0020C
| Computer (DB9F) | Via | UPS (DB9M) | Function |
|---|---|---|---|
| CTS 8 | direct | 2 | On Battery |
| DTR 4 | direct | 1 | Kill power |
| GND 5 | direct | 4 | Ground |
| GND 5 | direct | 9 | Common |
| RTS 7 | 93.5 kΩ resistor (or semiconductor) | 5 | Low Battery |
Signals
- Shutdown (UPS pin 1 → PC pin 4 DTR) — shutdown input to the UPS; the UPS shuts down when it receives a HIGH RS-232 level on this line for approximately 1 second (Back-UPS models). The shutdown only takes effect whilst the UPS is running on battery.
- AC Fail / On Battery (UPS pin 2 → PC pin 8 CTS) — output from the UPS; transitions from LO to HI RS-232 voltage level when the UPS switches to battery power.
- Common (UPS pins 4, 9 → PC pin 5 GND) — signal common / chassis reference for the UPS open-collector outputs.
- Low Battery (UPS pin 5 → PC pins 1 DCD / 7 RTS via transistor or resistor circuit) — open-collector output from the UPS indicating a low-battery condition; the 940-0020B revision uses a transistor circuit, whilst the 940-0020C revision uses an approximately 93.5 kΩ resistor.
Notes
- UPS pins 3, 5, and 6 are open-collector outputs rated for non-inductive loads up to 25 mA at up to +40 Vdc; pin 4 must be used as the common reference for these outputs.
- UPS pin 2 (AC Fail) transitions from LO to HI RS-232 level when the UPS is running on battery; it returns to LO when mains power is restored.
- The shutdown input (UPS pin 1) accepts a HIGH RS-232 level from the host DTR line; Back-UPS models shut down after approximately 1 second, whilst Back-UPS Pro and Smart-UPS models require approximately 6 seconds.
- Back-UPS models implement the Basic Port only; pins 7 (remote on/off) and 8 (+24 Vdc supply) that appear on Smart-UPS Advanced Port models are not present on Back-UPS.
- APC describes this port as proprietary; the cable is intended for use only with APC software and accessories.
- PowerChute Serial Shutdown cannot auto-detect a device connected via a simple signalling cable; manual installation must be selected during software setup.
- The 940-0020C revision reportedly includes circuitry that mimics mouse movement upon receipt of an on-battery signal, allowing the host computer to wake from sleep before initiating a shutdown sequence; the 940-0020B revision does not include this feature.
- Suffix letters (B, C, etc.) denote minor physical or OEM revisions only; all suffix variants of 940-0020 are electrically interchangeable.