The active layers of a 4-wire type touch screen consist of a partially conductive (resistive) coating applied uniformly to the panel. Conductive bus bars are screened with silver ink across opposing edges of the panel.

The ridged and flexible panels are mounted with the bus bars perpendicular to each other.

Measurements are made by applying a voltage gradient across one of the layers and measuring the voltage on the other layer. This measurement is made twice, once with the
gradient across the ridged layer and the measurement taken from the flexible layer and again with the gradient applied to the flexible layer and the measurement taken from the ridged layer.

The gradient is usually produced by grounding one bus bar and applying +5v to the other bus bar. This will produce a smooth voltage gradient in one axis across the panel.

With a 4-wire touch screen two separate setups and measurements are required, one in the X-axis (left-right) and one in the Y-axis (up-down) to define the touch point. Figure 2
illustrates the setup for making the two measurements. The actual switching of the 4 touch screen connections is done using an array of low on-resistance FET transistors and the
voltage measurements are made with an A/D converter. The host micro controller controls both these elements. When a layer is being used as a sens layer all other connections to
that layer must be left floating.
The resistance of the bus bars and the connection circuitry introduces an error (offset) in the voltage measurements.
These offsets can also drift with changes in temperature, humidity and time. If the touch screen is to be used only with a finger the offsets will constitute a small percentage of the voltage represented by the large size of the finger and can be ignored. If, however, the touch screen is to be used with a stylus for drawing or signature capture then these offsets should be taken into account. This can be done by calibrating the screen periodically or by utilizing an 8-wire touch screen.