User`s guide

5-6 Design

Issues

then out to the product's chassis or earth ground connection. If the node is floating

with respect to earth ground, the ESD current will return capacitively to earth via

the network wire, the power supply wires, and the PCB ground plane.

Designers of FTT-10A transceiver-based nodes should follow the PCB layout

guidelines presented in chapter 2. In addition, external clamping of user-

accessible circuitry is required to shunt ESD currents from that circuitry to the

center of the star ground on the PCB. For example, if the Neuron Chip in an FTT-

10A node is scanning a keypad with some of its I/O lines, then the I/O lines to that

keypad will need to be diode-clamped as shown in figure 5.3. If a negative ESD hit

discharges into the keypad, then the diode clamps to ground shunt the ESD current

into the ground plane. If a positive ESD hit discharges into the keypad, then the Vcc

diodes shunt the current to the ground plane via a 0.1µF decoupling capacitor that is

placed directly adjacent to the clamp diodes. The keypad connector, diodes and

decoupling capacitor should all be located close to the center of the star ground so

that the ESD current does not pass through sensitive circuitry on its way out of the

PCB.

+5V

Keypad

Neuron Chip

Circuitry

MMAD1103

Diode Array

(or BAV99/1N4148 Equivalent Diodes)

I/O

Lines

Figure 5.3 Illustration of I/O Line ESD Clamps

Vibration and Mechanical Shock Issues

The FTT-10A transceiver is designed to withstand moderate levels of vibration

and mechanical shock while operating. Echelon has tested the FTT-10A

transceiver operating in typical two-layer application boards to verify that the

transceiver can withstand occasional vibration levels up to 1.5g peak-to-peak

over the frequency range of 8Hz to 2kHz (where one "g" = 9.8m/s

), and

occasional mechanical shocks up to 100g.