Multiplexed Display - Character-oriented Displays

Character-oriented Displays

Most character-oriented displays (such as seven-segment displays, fourteen-segment displays, and sixteen-segment displays) display an entire character at one time. The various segments of each character are connected in a two-dimensional matrix and will only illuminate if both the "row" and "column" lines of the matrix are at the correct electrical potential. The light-emitting element normally takes the form of a diode so electricity will only flow in one direction, keeping the individual "row" and "column" lines of the matrix electrically isolated from each other. For liquid crystal displays, the intersection of the row and column is not conductive at all.

In the example of the VCR display shown above, the illuminated elements are the plates of many individual triode vacuum tubes sharing the same vacuum enclosure. The grids of the triodes are arranged so that only one digit is illuminated at a time. All of the similar plates in all of the digits (for example, all of the lower-left plates in all of the digits) are connected in parallel. One by one, the microprocessor driving the display enables a digit by placing a positive voltage on that digit's grid and then placing a positive voltage on the appropriate plates. Electrons flow through that digit's grid and strike those plates that are at a positive potential.

If the display had been built with every segment being individually connected, the display would have required 49 wires just for the digits, with more wires being needed for all of the other indicators that can be illuminated. By multiplexing the display, only seven "digit selector" lines and seven "segment selector" lines are needed. The extra indicators (in our example, "VCR", "Hi-Fi", "STEREO", "SAP", etc.) are arranged as if they were segments of an additional digit or two or extra segments of existing digits and are scanned using the same multiplexed strategy as the real digits.

A few character-oriented displays drive only one segment at a time. The display on the Hewlett-Packard HP-35 was an example of this. The calculator took advantage of an effect where very brief pulses of light are perceived as brighter than a longer pulse of light with the same time-integral of intensity.

Read more about this topic:  Multiplexed Display

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