the speed of an internal computer clock or to vary the  intensity  of  indicators  used  on  a  controlling device. DISPLAYS Some  computers  use  a  display  of  alphanumeric characters to provide computer status of the functional areas and for operator interfacing. You can find the interpretation  of  displayed  information  in  the technical manual or owner’s manual. The displays can be  used  when  the  computer  is  in  an  operational mode or a maintenance mode. Some computers use a dot  matrix  display;  others  use  light-emitting  diodes (LEDs)  to  display  the  alphanumeric  characters. Some  computers  use  a  small  three-  or  four-digit display to display an address and its contents. Other computers  use  larger  displays.  For  example,  one computer  has  a  large  display  consisting  of  up  to  44 alphanumeric characters per line and up to 6 lines. One portion of the display, when used for status, does not vary. The other portion, the operator interfacing part,   varies   in   accordance   with   the   types   of operations being run at the time. The technology used  with  the  operator  interfacing  portion  of  the display is ac plasma. This enables you to monitor operations. HOURS (TIME TOTALIZING METER) Time totalizing meters show the total number of hours power has been applied to a unit or module. They usually use a four-digit display to indicate the number of hours. The display is similar to the odometer of an automobile. INDICATORS (LIGHTS) The  simplest  way  to  show  the  status  of  an operation or a selected item is to turn on a light. Indicators   usually   come   in   several   varieties— backlit  indicators,  color  indicators,  and  clear  in- dicators. Backlit  indicators —The light bulbs are covered by a flat lens cover with clear alphanumeric cutouts that appear lit when the lamp is on. Color indicators —The light bulbs are inserted in a small solid colored casings with or without letters or characters. The whole casing glows when the lamp is lit. The casing can be square or round. 3-2 Clear indicators —Clear indicator are the same as the colored variety, but the casing is clear and usually round. PROTECTIVE DEVICES Protective devices can serve as controls. They are used in computers to prevent damage to the computer or to warn you of conditions that could be potentially dangerous.  Circuit  breakers  and  guards  are  two  ways we can protect the computer. Audible alarms are sometimes used to alert us to potential problems. Circuit  breakers —Circuit breakers remove ac input power when current becomes too high (i.e., internal short circuit). Guards —Guards are used to prevent accidental activation of selected keys and switches. A guard can be a clear lens that covers the key/switch or it can be a red cover. In either instance, the cover must be flipped up to gain unrestricted access to the key or switch. Audible   alarms  —Computers  often  use  an audible device, installed internally. These devices can provide a warning that an abnormal condition is about to take place, or they can sound an alarm when an abnormal condition is in progress. SWITCHES Switches are used to turn a unit on or off, to activate a function, or to set a parameter. You can activate switches by depressing them or flipping them up or down. Some switches are combined with an indicator to tell when they are activated. Switches  can  activate  an  immediate  response  in computer  operations. They can also be used to set parameters when the computer is being operated. Let’s take a look at the different types of switches associated with computers. They include key switches, position switches,  pushbutton  switches,  toggle  switches,  and two-position switches. See figure 3-1 for examples. Key Switches To activate key switches, you lightly depress the keys. Some keys are combined with an indicator. Two types are generally found on computer controlling units—alternate-action  keys  and  momentary-action/ contact  keys.