

From a recent BYTE magazine i got the following:

[Question and part of the answer deleted]

  If you are handy with a soldering iron, the loopback plugs are easy to
make.  On a serial RS-232 nine-pin port, use a female DB-9 connector and
connect pins 1 to 7 to 8; 2 to 3; and 4 to 6 to 9.  For serial RS-232 
25-pin ports, you'll need a female DB-25 connector with pins 1 to 7;
2 to 3; 4 to 5 to 8; 6 to 11 to 20 to 22; 15 to 17 to 23; and 18 to 25
connected.  To test a Centronics 25-pin parallel port, you'll need to
connect pins 1 to 13; 2 to 15; 10 to 16; 11 to 17; and 12 to 14 in a male
DB-25 connector.

					-Stan Wszola
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I haven't tried it. Use at own risk.