A power supply will generally shut off while the computer is running under the following conditions: Overtemperature  condition  (Two  overtem- perature conditions can occur. A  low overtem- perature condition provides a visual and/or audio warning that can be overridden with  battle  short switch. A high overtemperature  condition  will shut off the power supply.) Overvoltage or overcurrent condition Let’s  discuss  three  signals—power  interrupt, master clear, and stop. These are the signals a computer can use to provide protection. These signals or their equivalents are used in some computers that also have a pcb dedicated to monitoring power. They monitor ac line voltage to generate signals that allow orderly power start-up and power shutdown sequences. These signals can also be used to provide recoverability. POWER INTERRUPT (PI). —A PI is generated from the following conditions: Source power falls below specifications and returns to normal Source power is lost Computer set or cabinet is shut off A PI will generate a class I power interrupt; this is the highest priority of any CPU interrupt and cannot be locked out except by certain instructions. It alerts the software to a potential power loss. Logic power will remain to parts of the computer for an established time period  to  allow  the  software  to  prepare  for  the  potential power loss. The class I interrupt will give control of the CPU to a subroutine in memory. The subroutine will store  certain  CPU  registers  and  control  memory necessary for program restart. This allows the software to reestablish the conditions that existed before the PI occurred. MASTER CLEAR (MC), AUTOMATIC.  —An automatic master clear signal is generated a specific period after a PI occurs when the logic power falls out of tolerance and when power is lost or the computer set or cabinet is shut off. The MC signal is sent to all parts of the computer and will result in master clearing the CPU, I/O (including disabling acknowledgements in I/O, and main memory). The purpose of the MC signal is for a computer initialization after power has been applied. When the computer power comes to within specifications,  the  MC  will  be  released  and  control  will go to the auto-restart program if AUTO-START is 4-28 selected   on   a   controlling   panel.   Otherwise   the computer will be stopped in a cleared condition. STOP. —A stop signal is generated when the logic power goes out of tolerance. It occurs whether or not a PI signal is present and will send to memory to prevent any  new  memory  references.  The  purpose  of  this  signal is to prevent the loss of any memory data should logic power be lost faster than a normal turn-off sequence (PI or MC) can occur. Microcomputers Microcomputers  do  not  have  the  temperature requirements that mainframe and minicomputers have. They rely on the temperature of the room they occupy. They can, however, be affected by temperature if they are  run  when  the  room  temperature  is  too  high; generally  above  the  80°F  mark.  We,  therefore, concentrate our discussion on the power requirements. Figure   4-45   is   a   basic   block   diagram   of   a microcomputer’s power supply. It has the same basic components  as  mainframes  and  minicomputers. Microcomputers generate digital active signals out of the final stages to indicate that the power requirements have been met—one for ac and one for dc. These  signals  are  provided  to  the  backplane/ motherboard. Some computers have power supply LEDS on the backplane/motherboard to monitor the power  supply  output  voltages  and  the  power  supply status signals. If a problem exists in the power supply, these LEDs should indicate the problem by remaining off. The ac and dc status signals must be present to reset the computer. If equipped with power supply LEDs, they are used as part of the power-up diagnostic. AC SIGNAL. —A signal is sent to indicate that the ac input voltage is within specifications. If a minimum of 75 vac is applied to the input for at least 1 second, a signal  indicating  it  has  been  met  goes  active.  When  the input voltage drops to 60 vac or less, the signal goes low and remains low for at least 1 second. DC SIGNAL. —A signal is sent to indicate that the dc output voltages are within specifications. This signal goes active between 100 ms and 500 ms after the low-to-high transition of the ac signal. The dc signal remains active at least 5 ms (usually the minimum hold-up time for the dc outputs) after the high-to-low transition of the ac signal.