Published July 1999

THE ART AND SCIENCE OF RS-485

by Bob Perrin

Start • Arm Yourself • RS-485 101 • Getting Grounded • Shielding • Topology • Termination • Idle-state Biasing • Transients • Review Time • Sources

GETTING GROUNDED

Probably the least-understood issue associated with building robust RS-485 networks is proper grounding. Even though there are a number of good references on the topic, grounding seems to be misunderstood by many people [2, 3].

The common mode voltage (V_cm) is usually the parameter to be most concerned about. Figure 3 shows how V_cm is defined. TIA/EIA-485-A states, "Common-mode voltage (V_cm) is the sum of ground potential difference, generator (driver) offset voltage and longitudinally coupled noise voltage."

Figure 3—Common-mode voltage at the receiver depends on three parameters, two of which (Vnoise and VGPD) require attention by the engineer.

V_noise is coupled identically onto both wires. The result is usually referred to as common-mode noise. If a twisted pair is used, a useful simplification is to model V_noise as common mode.

V_GPD is the parameter that seems to cause the most problems. The problem stems from the oversimplification that ground is a perfect conductor capable of absorbing infinite energy, which is far from the truth [4, 5].

Earth ground potentials from circuit to circuit in an industrial installation can vary several volts under normal conditions. These voltage potentials most often stem from current leaking from equipment into the ground system.

However, during electrical activity (lightning, etc.), potentials between grounds in different parts of a building can momentarily reach tens or hundreds of volts depending on the geometry of the electric fields. Potentials between grounds in different buildings can even reach thousands or hundreds of thousands of volts [5].

The practical ramification of this is that earth ground is a poor choice for referencing signal grounds on distributed network nodes. The best method for controlling V_GPD is to simply run a third wire for the purpose of referencing local signal grounds. Figure 4a illustrates this process.

A less desirable but commonly used method for referencing local signal grounds is illustrated in Figure 4b. This method provides a higher impedance connection between nodes, which means local grounds may drift farther apart than with the scheme in Figure 4a. However, if the local supplies are not isolated or if ground loops are a concern, you can use the current-limiting mechanism shown in Figure 4b.

Figure 4c shows another variation of the scheme shown in Figure 4b. Earth ground is used as the third wire. V_GPD between nodes will vary as the earth ground potential varies across the network installation.

a)	b)
c)	Figure 4a—A dedicated conductor to reference signal grounds is the best method of controlling VGPD. b—The 100-ohm resistors limit current but allow larger VGPDs to develop. c—As a last resort, earth ground can be used to reference signal grounds. (Click here for larger graphics)

The common-mode voltage allowable between drivers and receivers on an RS-485 network is +12 to –7 V. This setup provides 7 V of protection from each rail (assuming a 5-V system). If the earth ground system in Figure 4c only varies a few volts under normal conditions, then the network will function fine.

The problem comes when a voltage transient appears on the earth ground circuit, which might happen because ESD is discharged into the earth ground near a node. Or it may happen because lightning strikes nearby (perhaps half a mile away). Whatever the cause, V_GPD between earth grounds on a network will occur on a daily or weekly basis.

When the common-mode voltage on a node drifts beyond the allowable V_cm of +12 to –7 V, the node is no longer guaranteed to function. In fact, the drivers and receivers in the node may be subject to damage. It’s up to the designer to protect the node from common-mode voltages beyond the silicon’s rating.

One useful part for this is a transient voltage suppressor (TVS). As I understand it, TranZorb is a registered trademark of General Semiconductor referring only to their line of TVSs. The widespread use of "TranZorb" to refer to all TVSs is a tribute to General Semiconductor’s early dominance in the market.

TVSs are silicon-based devices that utilize the nondestructive mechanism of avalanche breakdown to clamp high voltages. TVSs can be thought of as two back-to-back zener diodes that can momentarily dissipate hundreds or thousands of watts without ill effect.

Unlike metal oxide varistors (MOVs) and fuses, TVSs are not sacrificial components. With proper circuit design, TVSs can protect RS-485 networks indefinitely from momentary over-voltages.

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