This article details the serial connection pin requirements 

Serial Connection Pin Requirements

For touch connections, a straight-through pin-to-pin male to female serial DB9 9 pin cable is the standard (as in pin 1 of one end connected to pin 1 on the other end, pin 2 to pin 2, etc.).

Null modem serial cables (which cross-connect pins 2 and 3) will not work.

The minimum connection for serial when using the multi-touch Windows driver is pins 2 and 5, connected straight through.

The minimum connection for serial when using the single-touch 5-series Windows driver is pins 2, 3, 5, 6 and 8, all connected straight through.  The requirement can be reduced to pins 2 and 5 only by installing the driver using silent installation and including the /NoHandshake option.  See the User Manual in the Common section of the unzipped files for more detail.

When the single-touch driver loads, it sends a command (on pin 2) to the controller requesting identification. The controller sends a response on pin 3.  Pin 5 is ground, required to complete any circuit/signal path. When the controller is powered, it asserts pin 6 (DSR).  When the controller finishes its self-test, it asserts pin 6 (CTS).  By default the single-touch driver will not load unless those pins are asserted high (+ 5 vdc).