Figure  4-37.—Example  of  differential  point-to-point  line drivers  and  receivers  connected  in  parallel. Differential  drivers  and  receivers  —Differential drivers and receivers are used over long distances for high-speed communications. They are usually twisted pairs of wires or coaxial cables. Differential types can be point-to-point (fig. 4-37) or multiple source and destination (fig. 4-38). Single-ended  and  differential  line  drivers  and receivers are commonly used by some of the following interfaces: NTDS Input/Output (MIL-STD 1397) RS-232 (EIA RS-232) RS-422 (EIA RS-422) RS-449 (EIA RS-449) Line drivers  drive digital  information  over  both long and short distances to other equipments in a computer system. Line drivers do this by generating large voltage or current output digital signals from small voltage and current TTL or MOS digital signals to travel over transmission lines. Figure  4-38.—Example  of  differential  multiple  source  and destination  line  drivers  and  receivers. Line receivers  receive  digital  information  from both  long  and  short  distances  from  other  equipments  in a computer system. By the time the data reaches the line receiver from a line driver, the voltages are very low. The receiver operates with a very low threshold to detect these signals. Line receivers are usually used for long distances for parallel transmission. TOPIC 6—TIMING CIRCUITS Control  and  timing  circuits  comprise  a  very important area of a computer. A computer’s operations rely  on  commands/instructions  being  controlled (enabled  and  disabled)  at  specific  times.  Timing circuits are used to ensure the proper timing of enables and disables throughout the computer. Timing pulses are used to enable and disable specific circuits. This permits specific operations to begin and others to be ended. The return of these pulses a short time later could cause an enabled circuit to be disabled and another circuit to be enabled. In this way, operations previously begun are ended and anew set of operations is  started. When  a  program  is  installed  and  operating,  circuits are enabled and disabled through a sequential process that  continues  until  one  of  the  following  events  occurs: The program is completed A programmed stop is reached A fault condition occurs A pulse generator of a type determined by computer design provides the main timing signals for any given type  of  computer. These   pulse   generators   are commonly  termed  master  clocks  or  reference generators. They usually operate at a frequency or pulse repetition rate determined by the maximum rate at which the computer handles data. The master clock is the key to the timing circuits in the computer. It will set in motion the computer’s main timing circuits. From the main timing circuits, other timing circuits for the various other areas (arithmetic, memory, and I/O) can  be  enabled  or  disabled.  The  clock  will  produce electrical pulses with extreme regularity. The speed of the computer’s clock is determined by an oscillator. TIMING CIRCUIT COMPONENTS Timing  circuit  components  consist  of  wave generators  and  wave  shapers.  In  computers,  waveforms must be turned on and off for specific lengths of time. The time intervals vary from tenths of microseconds to several   thousand   microseconds.   Square   and 4-22