User`s guide

LONWORKS FTT-10A Transceiver User’s Guide 5-5

• ferrite beads in series with the network traces at the network connector, and

ferrite chokes in series with the power input traces at the power connector, can

be used to help meet EMC requirements for nodes that have noisy application

circuitry or special circuit requirements.

Early testing of prototype circuits at an outdoor EMI range should be used to

determine the effectiveness of these EMC techniques in a particular application.

ESD Design Issues

Electrostatic discharge (ESD) is encountered frequently in industrial and

commercial use of electronic systems

. In addition, the European Community

has adopted requirements for ESD testing

13.

Reliable system designs must consider the effects of ESD and take steps to protect

sensitive components. Static discharges occur frequently in low-humidity

environments when operators touch electronic equipment. Keyboards,

connectors, and enclosures may provide paths for static discharges to reach ESD

sensitive components such as the Neuron Chip and the FTT-10A transceiver.

This section describes the issues involved in designing ESD immunity into

FTT-10A transceiver-based products.

In addition to the following discussion, designers of FTT-10A nodes are strongly

encouraged to read references [10] and [12]. The EDN Designer's Guide to EMC

is especially helpful in understanding the importance of managing ESD return

currents.

Designing Systems for ESD Immunity

There are two general methods that are used to "ESD harden" products. The first

is to seal the product to prevent static discharges from reaching the sensitive

circuits inside the package. The second method involves designing the

grounding of a product so that ESD hits to user-accessible metal parts can be

shunted around any sensitive circuitry.

Since the network connector is user-accessible, it is not possible to totally seal

FTT-10A nodes. However, the product's package should be designed to minimize

the possibility of ESD hits arcing into the node's circuit board. If the product's

package is made of plastic, then the PCB should be supported in the package so that

unprotected circuitry on the PCB is not adjacent to any seams in the package. The

PCB should not touch the plastic of an enclosure near a seam, since a static

discharge can "creep" along the surface of the plastic, through the seam, and arc

onto the PCB.

Once an ESD hit has arced to the product, the current from the discharge will flow

through all possible paths back to earth ground. The grounding of the PCB and the

protection of user-accessible circuitry must allow these ESD return currents to

flow back to earth ground without disrupting normal circuit operation of the

Neuron Chip or other node circuitry. Generally, this means that the ESD currents

should be shunted to the center of a star ground configuration (see chapter 2) and