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Modem grounding: here's a new solution to a nagging problem.


You are probably familiar with the unpleasantness associated with an ungrounded piece of equipment.

A toaster with a broken ground wire can give you a pretty good shock.

The voltage hits your finger or hand because it is seaching for a return path.

To maintain uncorrupted electrical circuits for computer communication, a return path back to the source of the communication must be provided.

This insures that the potential, or voltage, of each piece of connected equipment is the same.

When the potential between equipment becomes unequal, safety is compromised and quality of communication degenerates.

Times Have Changed

Grounding may have been simple when data communications was young.

It grew as an issue as our communication technology became more complex.

In the early days of the telephone and telegraph, grounding was a simple issue.

The telegraph used the earth as a ground.

The operator's handset was attached to a battery, which powered it.

One side of the battery connected to the incoming signal line.

The other side was grounded through a rod planted in the earth.

The earth, with natural resistance of a few ohms, provided enough conductivity to satisfy the circuit's need for a return path.

Today's grounding problems are found between data communications equipment (DCE) such as multiplexers or a rack of modems, and data terminal equipment (DTE).

DCE and DTE system chasses are often still grounded to the earth.

This provides safety in case of a fault or electrical surge such as a lightning strike, and guards against disruptive potential differences between a modem rack and group of terminals.

But equipment in office buildings is usually at different distances from the earth.

This factor has created problematic differentials.

Garbled Data

This issue escalated as immense computer-room power systems developed.

In large computer systems, generating thousands of amperes, a minor electrical fault can create large voltage differences between equipment groups.

The more equipment distributed throughout an organization, the greater the risk of voltage drops, resulting in communication errors and garbled data.

To reduce the risk of shock to operators, equipment chasses are grounded to earth.

However, if both the DCE and DTE were grounded to earth, the potential difference caused by the varying length to ground would be carried through the signal connection, such as an RS-232 cable.

Although designed to handle a small variation in potential, large voltage differences could easily create data errors and damage the wire.

To solve this, a screw or wire jumper is used to separate the modem or DCE circuit from the frame ground, though keeping frame grounds and circuit grounds completely separate is difficult.

Let's say you have modem cabinet A and DTE B with DTE's grounds tied together and A's frame ground separated from the circuits.

If the jumper is not removed at A, the signal wires between A and B would try to carry the voltage created by any potential difference between A and B.

In a fault condition, the large chassis ground at B may be 100 volts and A may be 10 volts. The internal power grounds would try to carry the current caused by that 90-volt differential.

Now let's take a more complicated situation.

Modem rack A serves two groups of terminals, B and C.

Even though A has been isolated by removing the system ground, it does not isolate the ground between B and C.

B and C remain connected to A via the power ground in each circuit that runs through to A's power supply.

If B and C have different ground potentials, even though one may be the same potential of A, the power grounds would carry the voltage created by the potential difference.

This would disrupt the data and maybe damage the wiring.

The power supply in A has one common power ground for each circuit in the rack.

Potential differences between B and C flow through the power supply of A.

The outer shell of the card cage is isolated. The signals inside are subjected to the disruptive current flow.

New Technique

But there's a new development in grounding rack-mounted equipment.

Modem rack A supports B and C, each a separate group of terminals. Using a technique developed for our Rixon 4200 modem rack, each channel in the modem card cage is isolated with its own virtual power transformer.

No channel connects electrically to any other. The channel running to C maintains the same potential C gives as ground reference, regardless of B's potential.

Although different potentials may exist between the card slots in A, this never affects the user.

The chassis ground protects the outer casing.

With the chassis ground disconnected from the channel ground, no current is drawn through A's power supply. Each card operates as if powered by a dedicated transformer, with its own channel ground.

There is only one return path through each channel ground and no sneak path or short circuits.
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Altig, Richard
Publication:Communications News
Date:Feb 1, 1990
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