Using CMOS Circuitry – 4
■Serial and parallel - Customers sometimes connect the serial
and printer devices to the Micro PC while the power is off.
This can cause the failure mentioned in the above section,
Failure upon power-up. Even if they are connected with the
Micro PC on, there can be another failure mechanism. Some
serial and printer devices do not share the same power (AC)
grounding. The leakage can cause the serial or parallel signals
to be 20-40V above the Micro PC ground, thus, damaging the
ports as they are plugged in. This would not be a problem if
the ground pin is connected first, but there is no guarantee of
this. Damage to the printer port chip will cause the serial
ports to fail as they share the same chip.
■Hot insertion - Plugging cards into the card cage with the
power on will usually not cause a problem. (Octagon urges
that you do not do this!) However, the card may be dam-
aged if the right sequence of pins contacts as the card is
pushed into the socket. This usually damages bus driver chips
and they may become hot when the power is applied. This is
one of the most common failures of expansion cards.
■Using desktop PC power supplies - Occasionally, a cus-
tomer will use a regular desktop PC power supply when
bringing up a system. Most of these are rated at 5V at 20A or
more. Switching supplies usually require a 20% load to
operate properly. This means 4A or more. Since a typical
Micro PC system takes less than 2A, the supply does not
regulate properly. Customers have reported that the output
can drift up to 7V and/or with 7-8V voltage spikes. Unless a
scope is connected, you may not see these transients.
■Terminated backplanes - Some customers try to use Micro
PC cards in backplanes that have resistor/capacitor termina-
tion networks. CMOS cards cannot be used with termination
networks. Generally, the cards will function erratically or the
bus drivers may fail due to excessive output currents.
■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 switch onto a line, it creates a transient
waveform. Octagon has seen submicrosecond 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.
