You would think that a serial cable would be a straight-forward simple device that would be confusion- and mistake-proof. This is unfortunately not the case. The most common source of computer / mount communication failure is the wrong cable! There are straight-through cables and reversing cables. The reversing cables are also called crossover cables, null cables, modem cables, and null modem cables just to add to the confusion. Then there is every possible configuration of male / female plug - with locking screw or nut, without locking screw or nut. Many are not even labeled as to their orientation. (Maybe it isn't considered politically correct!) It is simply ridiculous. And the flunky at the big box computer store is no help.
Your mount MUST be connected with a straight-through cable. Straight-through cables are wired with the corresponding numbered pins of the connector wired to each other: pin #1 on the male end to hole #1 on the female end; #2 to #2 and so on. Other serial cable types cross or reverse some of the wires making communication with your mount impossible.
We have found a source of genuine standard straight-through serial cables and selected these 15 foot cables as the most universally appropriate. They have molded connectors, are fully shielded, and are lead-free for RoHS compliance (and all-around environmental friendliness). They also employ gold-plated contacts for superior data retention and corrosion resistance. They can even be used for monochrome or CGA/EGA monitor cable extension. Finally and most importantly: They actually work, and you don't have to drive all over town to find one! Need we say more?