Classic TrippLite Cable
This is the cable I hand-built to work with powerd.  It provides
power status sensing and inverter shutdown.

NOTE: The inverter shutdown provided via the serial transmit line
(TIOCM_ST).  genpowerd holds this line high by sending a BREAK
signal for the defined duration.  Due to the inflexibility of
this method, this serial line should only be used for killing the
inverter.

----- Begin Diagram -----

   9 pin to 25 pin  /-+--------+-\  25 pin to 9 pin cable
   +-\_____________/  | Patch  |  \________/-+
   |  _____________   | Panel  |   ________  |
   +-/             \  |        |  /        \-+
                    \-+--------+-/

Here is what the interface cable should look like:

   +---------------------------------------+-+--------------+
   |  UPS Side                             | |  System Side |
   +-----------------------------+----+----+-+----+----+----+
   |                             | 9  | 25 | | 25 | 9  |    |
   |Name                         |Pin |Pin | |Pin |Pin |Name|
   +-----------------------------+----+----+-+----+----+----+
   |                             | N/A| 01 | | 01 | N/A|FG  |
   | Line fail open/inverter off | 03 | 02 | | 04 | 07 |RTS |
   | Common Negative             | 01 | 08 | | 08 | 01 |DCD |
   | Inverter shutdown/(+) Side  | 02 | 03 | | 02 | 03 |TD  |
   | Inverter shutdown/(-) Side  | 04 | 20 | | 03 | 02 |RD  |
   +-----------------------------+----+----+-+----+----+----+

NOTE: The frame ground (FG) is not used on 9 pin serial cables
and was permanently attached on the break-out box I've been
using, so I included it -- I don't think it is functional.

NOTE: There has been some discussion as to the need of resistors
in the inverter shutdown circuit.  In the several months I used
this cable, I never used resistors in the circuit.  If you are