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1986-03-04
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Date: 26 November 1985
Number: 119
Information and Policy Statement Concerning Over-Voltage Protection
on "S" Server£
Novell has incorporated a design enhancement in the "S" line of
server products which makes them less susceptible to failure due to
induced high voltages on the transmission lines. The 6 port cluster
connector panel, part # 810-41-001, has been replaced with a 6 port
cluster connector panel with over-voltage protection, part #
810-102-001. The new "Intelligent NIC", part # 810-96-001, also
hasover-voltage protection circuitry, as does the regular NIC
810-86-001 Rev. D.
Novell has identified 3 modes of noise induction that can lead to
failure:
1.Common Mode Noise: Failures due to this type of noise represent
defective building wiring, or connecting inappropriate types of
equipment to the same circuit as the computing equipment. The third
wire grounding scheme is intended to guarantee that all equipment in
a building has the same ground reference. Properly designed
equipment is not supposed to send any current down the third wire.
Because the chassis ground (which must be connected tothe third wire
ground) is connected at some point to the logic ground,
current introduced into the third wire will also be able to flow
through the 47 ohm resistors between logic ground and the
transmission lines to the other machine and back to ground.
Currents larger than 0.1 amp were able to destroy the transmitters
and receivers prior to the new design change. Failure due to this
type of noise represents a defect in the buildings wiring or a
defect with equipment in the building. It can be corrected by
properly wiring the building. It can be detected by monitoring
voltage between the two chassis ofthe connected equipment.
2.Local Ground Differences: Ground reference is a local phenomenon.
The potential difference between two rods driven into the gound a
few hundred yards apart can be substantial. During storms it can be
thousands of volts. Because any particular building is generlally
connected to a single grounding system, problems due to this
phenomenon generally occur when connecting between buildings.
When a potential difference exist between ground references of two
connected systems, current will tend to flow between the machines
overthat connection with the potential for destroying components if
this current value becomes significant. Proper design of the
connection between separate grounding systems can reduce or
eliminate failures due to this type of problem. Connections between
buildings are best accomplished with a conductive conduit(aluminum or
other metal) which is connected to the respective groundingsystems
at both ends and which have rods driven into the ground at 20
foot intervals and connected to the conduit along its entire length.
Cables runthough this conduit, and equipment connected to the two
respective grounding sytems, will receive substantial protection from
this scheme. This type of problem can be detected by monitoring the
voltage between the chassis of thec onnected equipment. With the
"S" system this voltage can be monitored between the shield of the
cable and the chassis of the PC as the shield of a properly made
cable is connected to the chassis of the server but not to the
PC.
3.Electromagnetically Induced Noise: The connection between the
server and the PC along with the connection of the server and the
PC to the ground form aloop which can have currents induced into it
via capacitance and inductance. The shielding sheme of the server is
quite effective against capacitively induced noise but not nearly as
effective against inductively coupled noise. Its effectiveness as a
shield protecting against component failures would been hanced by
connecting the shield at both ends instead of at the server only, but
then the shield could become a source of interference for the
transmitted data. Inductively induced noise is the main type of noise
the new protective circuits are designed to protect against. Observed
failures due to this type of noise have been largely attributable to
poorly placed transmission cables. Cables run outside of buildings
without properly grounded protective conduitare very susceptible as
are cables in wiring troughs with power mains. Again,proper cable
routing eliminates most failures from this type of noise.
The over-voltage protection on the new boards is expected to provide
substantial protection from all 3 conditions as it routes damaging
levels of electrical energy directly to the chassis. Components on
the new boards are protected against transients to hundreds of times
previoius safe levels. Steady-state safe levels are increased an order
of magnitude.
MIS-INFORMATION
There has been some confusion about the nature of the connections
to the "S" server. The cabling for the "S" server connections contain
no ground wires. The shield is connected directly to the chassis via
pins 1, 2, and 3 which are tied tothe mounting hardware fastening to
the rear panel, which is the chassis.
Ordering Information
The 6 port cluster connector panel with over-voltage protection is
available at asuggested retail price of $100.00, and its part number is
810-102-001. Novell feels this is a product enhancement, and that in
almost all cases appropriate cabling practices will eliminate individual
port failures.
NIC cards of the 810-56-00X and 810-86-001 (REV. C and earlier)
series can haveover-voltage protectors added to them for $50.00 plus
shipping from Novell CustomerService. Please obtain appropriate RMA
numbers prior to sending any material to Novell.
The newer "Intelligent NIC", part number 810-96-001 and regular NIC,
part number 810-86-001 Rev. D, already has over-voltage protectors
built in, so there is noneed to have any added to these boards.
Disclaimer
No amount of protective circuitry can protect against improper
installation. There will be conditions that cannot be protected
against, for example, a direct lightning strike can easily induce
voltages which will destroy a system. There will be no protection
from boards with the over-voltages protectors if the screws
connecting those boards to the chassis are not installed.
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