Echelon FT 3150 Smart Transceiver - Manual (Page 108)

Chapter 4 - Hardware Design Considerations
102 FT 3120 / FT 3150 Smart Transceiver Data Book
into the ground plane. If a positive ESD hit discharges into the keypad, then the V
CC
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.
Figure 4.7 Illustration of I/O Line ESD Clamps
Lightning Protection
Protection against lightning is required when designing control networks that run outside of buildings.
Building Entrance Protection
Echelon recommends using shielded twisted pair wire for all networks, or portions of networks, that are run outside
of buildings or grounded structures. The shield, as well as the two network lines, should be connected via Data Line
Lightning/Surge arresters to earth ground at each building entry point, to conduct excessive energy surges or
lightning strike energy directly to earth and prevent their entry inside the building via the network shield and/or data
line conductors. Therefore, three arresters are to be used at each building entrance in the case of Shielded Twisted
Pair wiring.
Network Line Protection
The arresters used on the network data lines must be of the Gas Discharge type. The intrinsically low capacitance to
ground of these devices, typically less than 5 pF, minimizes the corruption of any data signals. Due to their low
capacitance construction, the use of Gas Discharge devices does not alter the maximum number of nodes allowed per
network segment, i.e., 64 for unpowered TP/FT-10 segments, 128 for link powered TP/FT-10 segments. MOV and
TVS protection devices must NOT be used on the network data lines due to their much higher capacitance (>200
pF) and potentially poor differential capacitance matching. These devices may corrupt, and possibly prevent,
network communication between nodes.
+5V +5V
Keypad
Smart Transceiver
Circuitry
MMAD1103
I/O
Lines