How to Create a UPS Cable for the APC Smart-UPS

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Article ID: 102711 - View products that this article applies to.
This article was previously published under Q102711
The APC Smart-UPS series models 900, 900XL, 1250, and 2000 systems include a standard RS-232 line failure output and UPS shutdown input. The low battery signal, however, is not a standard RS-232 signal and requires special configuration for use with Windows NT.

For RS-232 outputs and inputs, the line failure, low battery, and UPS shutdown signals can be connected from the UPS to one of the computers RS-232 (serial) ports. Because the low battery signal generated by the APC Smart-UPS is an open collector output, it must be pulled up to a common reference voltage by means of a pullup resistor.

Following is the configuration for a cable that can be used with the APC Smart-UPS series, including the implementation of the pullup resistor.
   DB9 9 Pin Female Connector       DB9 9 Pin Male Connector
    (to PC serial port)              (to UPS connector)
          Pin #5------ Ground ----------Pin #4
          Pin #8------ Power Failure--- Pin #2
          Pin #4------ UPS Shutdown---- Pin #1
          Pin #1------ Low Battery----- Pin #5 --[10k ohm]-- Pin #8
The above diagram indicates that Pin #5 on the PC serial port should be connected directly to pin 4 on the UPS side, pin 8 to pin 2, pin 4 to pin 1, and pin 1 to pin 5. In addition, pins 5 and 8 on the same side (the UPS side) have a 10k ohm (10,000 ohm) resistor connected between them. This resistor serves to pull the voltage on this non-RS-232 output high until an actual low voltage is generated by a low battery condition.

With the above cable, the following polarities should be set in the UPS section of the Control Panel:
   Power failure signal: Positive
   Low battery signal:   Negative
   Remote UPS shutdown:  Positive
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American Power Conversion Smart UPS 900, 900XL, 1250 & 2000 Owners Manual

The APC Smart-UPS is manufactured by American Power Conversion, a vendor independent of Microsoft; we make no warranty, implied or otherwise, regarding this products' performance or reliability.


Article ID: 102711 - Last Review: October 31, 2006 - Revision: 2.1
  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows NT Workstation 3.1
  • Microsoft Windows NT Advanced Server 3.1
kbhardware KB102711

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