The seven-segment LED display has four individual digits, each with a decimal point. Each of the seven segments (and the decimal point) in a given digit contains an individual LED. When a suitable voltage is applied to a given segment LED, current flows through and illuminates that segment LED. By choosing which segments to illuminate, any of the nine digits can be shown. For example, as shown in the figure below, a 2 can be displayed by illuminating segments a, b, d, e, and g.
Seven segment displays come in two varieties - common anode (CA) and common cathode (CC). In a CA display, the anodes for the seven segments and the decimal point are joined into a single circuit node. To illuminate a segment in a CA display, the voltage on a cathode must be at a suitably lower voltage (about .7V) than the anode. In a CC display, the cathodes are joined together, and the segments are illuminated by bringing the anode voltage higher than the cathode node (again, by about .7V). The Digilab board uses CA displays.
The seven LEDs in each digit are labelled a-g. Since the Digilab board uses CA displays, the anodes for each of the four digits are connected in a common node, so that four separate anode circuit nodes exist (one per digit). Similar cathode leads from each digit have also been tied together to form seven common circuit nodes, so that one node exists for each segment type. These four anode and seven cathode circuit nodes are available at the J2 connector pins labelled A1-A4 and CA-CG. With this scheme, any segment of any digit can be driven individually. For example, to illuminate segments b and c in the second digit, the b and c cathode nodes would be brought to a suitable low voltage (by connecting the corresponding circuit node available at the J2 connector to ground), and anode 2 would be brought to a suitable high voltage (by connecting the corresponding circuit node available at the J2 connector to Vdd).
The Digilab board uses two 2-digit displays to create a single 4-digit display. These displays use the reference designators DSP1 and DSP2, and they appear as relatively large rectangular boxes on the silk screen. Since they contain LEDs, they must be loaded into the board with the correct orientation or they will not function - the displays must be loaded with the decimal points nearest the slide switches.