Octagon 5420, User Manual

Octagon has never found a single case where multiple IC
failures were not caused by misuse or accident.  It is very
probable that multiple component failures indicate that they
were user-induced.

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Testing “dead” cards - For a card that is “completely
nonfunctional”, there is a simple test to determine accidental
over voltage, reverse voltage or other “forced” current
situations.  Unplug the card from the bus and remove all
cables.  Using an ordinary digital ohmmeter on the 2,000 ohm
scale, measure the resistance between power and ground.
Record this number.  Reverse the ohmmeter leads and
measure the resistance again. If the ratio of the resistances is
2:1 or greater, fault conditions most likely have occurred.  A
common cause is miswiring the power supply.

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Improper power causes catastrophic failure - If a card
has had reverse polarity or high voltage applied, replacing a
failed component is not an adequate fix.  Other components
probably have been partially damaged or a failure mechanism
has been induced.  Therefore, a failure will probably occur in
the future.  For such cards, Octagon highly recommends that
these cards be replaced.

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Other over-voltage symptoms - In over-voltage situations,
the programmable logic devices, EPROMs and CPU chips,
usually fail in this order. The failed device may be hot to the
touch.  It is usually the case that only one IC will be
overheated at a time.

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Power sequencing - The major failure of I/O chips is caused
by the external application of input voltage while the Micro PC
power is off.  If 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.

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Failure on power-up - 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 power-up.  This type of failure is
typical on serial interface chips.