APC Back-UPS Cable (PN 940-0020B) pinout

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DB-9 Male|DB-9 Female|Signal Name
(UPS end)|(PC end)   |
---------|-----------|-----------
  N/C    |     1     | N/A
  N/C    |     2     | N/A
  N/C    |     3     | N/A
   1     |     4     | Data Terminal Ready
  4,9    |     5     | Signal Ground*
  N/C    |     6     | N/A
  N/C    |     7     | N/A
   2     |     8     | Clear To Send
  N/C    |     9     | N/A

* There are two different configuration, one in which both 4 and 9 on
the male end are connected to 5 on the female end, and one in which only
4 on the male end is connected to 5 on the male end.