tedbear - 4/3/2009 02:52

The general rule that I use is the following: If the sex of the two devices is opposite (they will mate) then a standard cable will do. The only purpose of the cable is to gain length.

If on the other hand the two devices are the same sex (both male or both female) then a null modem gender changer is needed. The null modem gender changer could be a M-M such as used with a standard GPS going to an Ag Leader product or a F-F which is needed in other situations.

Like any rule - my rule has exceptions but is generally correct. Fortuately with the exception of connecting to Pin 4 of an Ag Leader device, there is generally no damage done with the wrong cable. You just experience high frustration because the units do not work together as you desire.

To avoid problems I recommend clipping all pins but 2,3 & 5 when using generic cables to gain length. If the cable is a straight cable 2-2, 3-3 and 5-5 should be observed. If the cable is a null modem type then 2-3, 3-2 and 5-5 should be observed.

This whole business is rooted in history. In the early days, teletype equipment was generally hooked together over a phone modem. A phone modem connected the Tx of unit A to the Rx of unit B so that A could talk to B. Likewise the Tx of unit B was connected to the Rx of unit A so that B could talk to A. There were some other lines that did the crisscross thing. You might say these were messages from one unit to the other asking things such as "are you ready for more data"? Then the other unit might respond with "yes go ahead". Again this was handled by the phone modem.

Suppose these two devices were in the same room and no phone modem was going to be used. You would want the female connector of unit A to connect to the female conector of unit B. This would require a cable that was M-M and also crisscrossed the Tx and Rx wires. This is the origin of the word null (not) modem terminology. For GPS equipment the Rx and Tx lines are generally the only ones required for communication as the "are you ready" thing is rarely used.