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
Lets discuss three signalspower 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:
l Source power falls below specifications and
returns to normal
l Source power is lost
l Computer set or cabinet is shutoff
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
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
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 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
microcomputers 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 metone 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.