Knowledge Base

Capacitive

A capacitive touch screen consists of a glass panel with a capacitive (charge storing) material coating its surface. Circuits located at corners of the screen measure the capacitance of a person touching the overlay. Frequency changes are measured to determine the X and Y coordinates of the touch event.

Capacitive type touch screens are very durable and have a high clarity. They are used in a wide range of applications from restaurant and Point-Of-Sale use to industrial controls and information kiosks.

The Latitude XT is an example of a system that uses capacitive touch.

Resistive

The resistive technology is acheived by using two layers The fisrt is a hard-coated layer of ITO (indium tin oxide). The second is a layer of conductive clear glass. These two layers are held apart by spacers. When the flexible (ITO) layer is touched and contacts the other layer of glass, the voltage is detected and measured. That same voltage is applied as an electrical field. The controller uses voltage measured from along the X-Y axis to calculate the position of the touch.

The Latitude XFR D630 is an example of a system that uses resistive touch.

Digitizer

The digitizer is a key component to tablet PC technology. Essentially it is a transparent grid which rests on top of the LCD. It calculates the coordinates of the input device (pen or touch) and converts that signal into visible writing.

The block diagram below demonstrates the layout of the digitizer components. The a-trig sensors occupy the X and Y axis of the digitizer's borders. The d-trig is the controller that connects to the USB bus via the LVDS cable.

Digitizer Troubleshooting

The pen is the first component to be investigated in the event of a suspected problem with the digitizer. Verify the tip is in good shape (free of chips, excessive wear, etc.) by closely examining it. If there is any doubt, you should change out the tip with a new one or one that is known to be in good condition.

You should also verify that the touch capabilities are not affected. Switch to touch mode and see if the problem still exists. If there are no symptoms present while in touch mode, the pen tip is the most likely suspect. If the problem does persist in touch mode you should run diagnostics and take the necessary steps depending on the results.

Only the diagnostics built into the N-trig Tablet Settings applet are valid for testing the digitizer. The Dell 32-bit and PSA+ diagnostic routines do not test the functionality of the digitizer.