For buses, this disadvantage is outweighed by the need (compared to manual) to change each destination separately if changing routes, this could be up to seven different blinds. Although these sign systems are normally accurate, over time the blind becomes dirty and the computer may not be able to read the markings well, leading occasionally to incorrect displays. The on-board computer is normally programmed with information on the order of the displays, and can be programmed using the non-volatile memory should the blind/roll be changed. Barcodes are printed on the reverse of the blind, and as the computer rolls the blind an optical sensor reads the barcodes until reaching the code for the requested display. These signs are controlled by a computer through an interface in the driver's cabin. Automatic changing of rollsign/blind displays, through electronic control, has been possible since at least the 1970s, but is an option that primarily has been used on rail systems-where a metro train or articulated tram can have several separate signboxes each-and only infrequently on buses, where it is comparatively easy for the driver to change the display.
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