Octagon Systems X-COM-2 Reference Manual page 4

you apply 5V to the input of a TTL chip with the power off, nothing will
happen. Applying a 5V input to a CMOS card will cause the current to
flow through the input and out the 5V power pin. This current attempts
to power up the card. Most inputs are rated at 25 mA maximum. When
this is exceeded, the chip may be damaged.
Failure on powerup: Even when there is not enough current to destroy
an input described above, the chip may be destroyed when the power to
the card is applied. This is due to the fact that the input current biases
the IC so that it acts as a forward biased diode on powerup. This type of
failure is typical on serial interface chips but can apply any IC on the
card.
Under rated power supply: The board may fail to boot due to an
under rated power supply. It is important that a quality power supply be
used with Octagon Systems cards that has sufficient current capacity,
line and load regulation, hold up time, current limiting, and minimum
ripple. It is extremely important to select a supply that ramps up in
10ms or less. This assures that all the circuitry on the CPU cards
sequences properly and avoids system lockup.
Excessive signal lead lengths: Another source of failure that was
identified years ago at Octagon was excessive lead lengths on digital
inputs. Long leads act as an antenna to pick up noise. They can also act
as unterminated transmission lines. When 5V is switched onto a line, it
creates a transient waveform. Octagon has seen sub-microsecond pulses
of 8V or more. The solution is to place a capacitor, for example 0.1 µF,
across the switch contact. This will also eliminate radio frequency and
other high frequency pickup.
4