I don't think I've ever made up a custom serial
cable that was right the first time. On the DB9 type, pins 2 and 3 are your data pair and pin 5 is typically a common ground, which may or may not be separate from other grounds.
The problem is that pins 2 and 3 are described as "TxD" and "RxD" and those descriptions ARE WORSE THAN USELESS because they are relative terms and the descriptions are REVERSED depending on which side of the cable you are working on.
For instance, the "TxD" line from a GPS
is the line on which the GPS
is "Talking". From the GPS maker's point of view, that is TxD. Now if you go to plug
that into an autopilot--you are going to plug it into the RxD connection, because the autopilot
is LISTENING for that talker on that wire. TxD always becomes RxD, and RxD always becomes TxD, when all you have is a simple one-to-one connection.
In more complicated setups, where there may be several "talkers" and one "listener", or vice versa, how you wire things up may change.
Bottom line? Tx and Rx (as they are also commonly called) should be on pins 2 and 3, and if it doesn't work, one of the most common problems is that you've just got these backwards. Me, sometimes I get boat brains and wire the DB9 up backwards anyway. I find that it really really helps to use a loupe, make sure of which pins are #2 and #3, and then MARK THEM with a paint
or magic marker, before even thinking about soldering them. It is very easy to rotate the DB9 shell in your hand, and wind
up with the pins connected wrong.