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Motherboards are designed for numerous configurations. Some
configurations may want single point ground located
elsewhere. Some configurations may attach the entire
motherboard to chassis plate, and insulate chassis plate so
that single point ground is between chassis plate and
chassis. Furthermore, all mounting holes must be plated
through. Computer assembler is expected to bring basic
electrical knowledge to the assembly. He - not the
motherboard manufacturer - decides where the single point
ground will be made. Motherboard manufacture makes products
with numerous options. Computer assembler decides how to
implement those options.
Cross talk and other problems are why computer boards are
multi-layer assemblies using a copper plane for ground and
other planes for Vcc. Same problems also why Intel (not the
PC board designer) defines exact copper layout for PC traces
to CPU. The layout is that critical. Electronic nature even
of copper is but one reason why a CPU power supply is adjacent
to the CPU and why newer ATX power supplies have a second
power cable.
Measure the conductivity between too ends of that
motherboard ground plane. To meter, DC voltage is a perfect
short. In reality, electric currents are RF electricity. No
copper - even the ground plane - is a perfect conductor. All
copper has significant impedance. Impedance (not resistance)
is why Intel provides a layout for connections to their CPUs.
Impedance is why bypass capacitors are scattered all over the
motherboard. Conductors are electronic components when
discussing RF currents to computers and in external generated
transients. RF is why copper is not truly conductive.
If copper were truly conductive, then antennas would not
transmit and receive. Best antennas have near zero resistance
and a specific impedance. The impedance (not resistance)
makes multiple grounding of a motherboard problematic. Just
another reason why grounds are at a single point.
All conductors are electronic components. As Gary notes:
... at the frequencies modern computer systems operate at they
have to use multiple grounds otherwise the systems become
inoperable/unreliable because of cross talk.
Problems are not just limited cross talk. Cross talk alone
is not the reason for all those bypass capacitors. All PC
traces and even the PC board ground plane are electronic
components. Some currents (ie static electricity) through a
ground plane can cause catastrophic consequences especially in
hospital equipment. All grounds must be interconnected. So
again, the interconnections between various grounds use the
single point concept. Hospitals take the concept even one
step farther. Hospitals do single point grounding on a room
and on a building wide level.
Bottom line for computer assemblers who learn new tricks - a
single point connection between motherboard and chassis plate
is best adjacent to IO slots and power connection. This post
demonstrates another technical reason why: all conductors are
electronic components; especially when dealing with RF
frequencies in computers and in static electric discharges.
Just another reason why more reliable computers uses the
single point grounding technique.
"We never did that before. Therefore we don't have to do
that." People with this mentality need not reply. Unlike
other posts that disagree, these posts technically explain
why, provide examples, and provide experiments to demonstrate
the problem. IOW these posts are based upon the science and
not upon the myths so common among computer assemblers. These
posts are for those who keep learning new techniques. Single
point grounding of motherboard to chassis is the preferred
mounting method. Provided here is but another reason why: RF
currents make copper an electronic component. The relevant
parameter is impedance.
Gary wrote:
I read the article and it mainly deals with analog devices
and/or A/D
converters and low frequency PIC chips. While it is true that multiple
grounds can create ground loops creating hum in audio or telephone
circuits we're dealing with radio frequencies in the hundreds of
megahertz for the system bus / memory and in the gigahertz region in
the case of the processor.
What the article didn't address and is more of a concern to
motherboard manufacturers is cross talk. One example of cross talk is
noise from one set of data / memory lines at 100/133/400 megahertz
bleeding over onto another set of data / memory lines. In computer
circuitry this is disastrous. What may be a ground at DC or in low
level audio or radio circuits can be a great antenna for radiating
noise at 100+ megahertz. I'm an amateur radio licensee and I've got an
antenna that's at DC ground potential. If you measured the resistance
between the two leads to it, its a dead short and would be a short
even up to the range of several megahertz. But if is a great antenna
at 144 thru 148 Megahertz. ( Our 2 meter VHF band )
I've gone over this years ago with another person and I did a search
and found a web site of a computerized medical equipment manufacturer
( not too much room for error there ) and they stated just what I
have above and that is at the frequencies modern computer systems
operate at they have to use multiple grounds otherwise the systems
become inoperable/unreliable because of cross talk. The late 70's era
Radio Shack Model 1's expansion interface was a classic example of
cross talk. It was housed in a plastic case and radiated almost as
much cross talk inducing noise as a radio station and was notoriously
unstable. It only ran at around 1 megahertz if memory serves me
correctly.
You'd also have to ask yourself if motherboard manufacturers didn't
want the motherboards to be grounded to the brass standoffs why do
they conveniently put a couple of soldered circles right in the exact
spot where those standoffs are ? As an experiment you can take and
attach the power leads part way onto the motherboard ( so the tips of
your ohmmeter can connect to the metal surfaces ), then take an
ohmmeter and measure the resistance between those soldered circles and
the ground wires and you'll find that it's a dead short, at least it
was on any motherboard I've ever checked.
HTH
Gary