This article details which touch connection to use. 

Some Elo touch monitors include both a USB touch interface and a DB9-RS232 Serial connection. A common question we receive is:

“Should I connect both touch cables?”

No. Only one touch interface should be connected—either USB or RS232, not both. Connecting both may cause the touchscreen to malfunction or not respond properly.

For most Windows systems, we recommend using the USB connection, as it provides plug-and-play support and better compatibility. For legacy systems that do not support USB, use the RS232 Serial connection instead.

Note: Do not confuse the RS232 Serial connection with the 15-pin VGA video connection. Although they appear similar, the RS232 connector has 9 pins, while VGA has 15 pins and is used for video, not touch.