LM78 is a hardware monitoring chip made by
National Semiconductor. It can monitor 5
positive voltages, 2
negative voltages, 3 fans speeds, a case intursion sensor, and a
CPU temperature sensor. The lm78 measures voltages between 0 and 4.8
volts, so readings for the +5 and +12 rails are divided down, measured, the scaled by the software that monitors the lm78. Most computers built during or after 1998 have an lm78 or similar chip in them. An lm78 can interface with the system via the
ISA bus or the
SMBus (System Managment Bus)
The lm78 also supports alarms that can trigger an
interrupt when a sensor goes out of a specified range. Most users, however, don't have a spare
IRQ line to allocate to the lm78, and some
motherboard makers don't even bother connecting the lm78's interrupt pin.
Software to monitor these chips is available for
M$ Windows and
Linux (via the
lm_sensors kernel module). There are applets for just about every
window manager that will display
CPU temperatures and voltages on the screen.
full documentation of the LM78 chip is available on national semiconductor's web site at http://www.national.com