The Tripp Lite OmniPro 450 and OmniPro 675 are line-interactive UPS systems from the late 1990s that present a DB-9 female port on the rear panel for computer communication. This cable variant provides contact-closure signalling compatible with the Windows NT UPS Service, which monitors power status via RS-232 handshake lines rather than a serial data protocol. The UPS closes relay contacts to indicate mains failure and low-battery conditions, whilst the PC asserts DTR to trigger a controlled shutdown. Tripp Lite did not publish pin-level wiring tables in the OmniPro owner’s manual; a proprietary cable is required, as standard straight-through and null-modem cables are not suitable.
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 |
|---|---|---|---|
| Shutdown | 2 | 4 | DTR |
| PWR Fail | 5 | 8 | CTS |
| LOW BATT | 6 | 1 | DCD |
| Common | 1,4 | 5 | GND |
Notes
- Pin 7 at the PC end (RTS) connected by R1 2k0 to pin 8 (CTS) and connected by R2 2k0 to pin 1(DCD)
- Power_Failure - Negative, Low_Battery - Negative, UPS_Shutdown - Positive.
Signals
- Shutdown — driven positive by the PC’s DTR line (pin 4) to command the UPS to shut down output power.
- PWR Fail — contact-closure output from UPS; active-negative on CTS (PC pin 8); closes when mains supply is lost.
- LOW BATT — contact-closure output from UPS; active-negative on DCD (PC pin 1); closes when battery is near exhaustion.
- Common — signal ground reference; UPS pins 1 and 4 connected together to PC pin 5 (GND).
- RTS bias — PC pin 7 (RTS) is not connected to the UPS; it is tied via R1 (2 kΩ) to CTS and via R2 (2 kΩ) to DCD to provide a positive voltage bias rail, ensuring the NT UPS Service reads the correct idle state on those inputs.