AUTOMATIC  BUS  TRANSFERS  (ABT’S).— ABTs  are  devices  that  transfer  primary  power  from  one source to another in a minimal amount of time. Some computer  power  supplies  can  accommodate  this feature.   This   allows   the   computer   to   continue executing  software  during  ABTs  or  other  power absences of up to 100 ms. STANDBY  POWER  SUPPLY  (SPS).—  A standby power supply uses special circuitry that can sense the ac line current. If the sensor detects a power loss on the line, the system quickly switches over to a standby  battery  and  power  invertor.  The  power  invertor converts the battery to ac power, which is supplied to the  computer. UNINTERRUPTIBLE  POWER  SUPPLY (UPS).— An  uninterruptible  power  supply  provides power   that   is   completely   uninterruptible.   It   is constructed in much the same way as an SPS with the exception of the switching circuit. Your computer is running  off  a  battery;  therefore,  no  switching  takes place  and  no  system  disruption  takes  place.  If  equipped as such, after a specified time period, a diesel-powered generator is automatically started; this conserves the battery. SUMMARY—COMPUTER COMPONENTS AND CIRCUITS This chapter has presented material on computer components and circuits. These include computer number  systems,  computer  logic,  and  integrated circuits, both digital and linear. It also has presented information on the functions of circuits, the types of data and formats, and the power supplies used by computers.  The  following  information  summarizes important points you should have learned: COMPUTER NUMBER SYSTEMS— Digital computers use derivatives based on binary numbers. The two most commonly used are the octal and the hexadecimal  number  systems.  These  number  systems are used for functions and mechanization of logic circuits. INTEGRATED CIRCUITS (ICs)—  Computers rely heavily on ICs. An integrated circuit is a complete electronic circuit, containing transistors and perhaps diodes,  resistors,  capacitors,  and  other  electronic components,  along  with  their  interconnecting  electrical conductors. The types of integration are small-scale (SSI), medium-scale (MSI), large-scale (LSI), and very large-scale (VLSI). ICs are packaged in many ways. IC FAMILIES— The IC families are bipolar and metal-oxide semiconductor (MOS). They can both be used  in  digital  and  linear  ICs.  They  can  also  be combined and are called bipolar MOS (BIMOS). DIGITAL IC’S— Digital ICs handle information by means of switching circuits. They are used to process and store information. The basic building blocks of digital logic circuits contained in a computer are logic gates. The logic circuits contained in digital logic  circuits  are  classified  as  combinational  digital circuits and sequential digital logic circuits. LOGIC GATES— Three logic gates are the basis for all logic gates. They are the AND, OR, and NOT logic gates. These three logic gates are used indifferent combinations and variations to form logic gates that perform  decision-making  functions  throughout  the computer. FLIP-FLOPS—  Flip-flops  are  sequential  logic elements. They have only two output states, either 0 or 1. Flip-flops controlled by a timing signal (commonly called  the  clock  pulse)  are  called  synchronous operations. Flip-flops not controlled by timing are called asynchronous operations. Other flip-flops have gated  (latched)  operations.  This  means  the  logic function is turned on or off dependent upon an input control signal (command or enable). FUNCTIONAL   USES   OF   DIGITAL IC’S— Digital ICs may be used for decision-making functions. These include code converter circuits and data   routing   circuits. They  are  also  used  for memory-type   functions. LINEAR IC’S— Linear ICs are amplifying-type circuits in integrated form. They are regulating as opposed  to  switching.  The  output  of  a  linear  circuit changes in a smooth, even manner as the input is changed at a constant rate, so that a graph of output versus input is approximately a straight line; hence the name  linear. Linear  ICs  use  bipolar  and  MOS technology.  The  basic  gate  for  a  linear  IC  is  the operational amplifier (op amp). FUNCTIONAL USES OF LINEAR IC’S— In computers,  linear  ICs  are  used  as  general  linear  circuits to perform amplifying functions inside the computer. They are also used as system interface circuits to amplify  data  signals  entering  and  leaving  the  computer or internal parts of the computer. TIMING CIRCUITS— Timing circuits are used in a computer to ensure the proper timing of enables and disables throughout the computers. Timing circuit 4-30