User`s guide

2-10 Electrical
Interface
5. Ground Planes: As ground is routed from the center of the star out to the function
blocks on the board, planes or very wide traces should be used to lower the inductance
(and therefore the impedance) of the ground distribution system.
6. +5V Power Distribution &
Decoupling: In general, Vcc should be distributed
through low inductance traces and planes in the same manner as ground. At least
two 0.1µF SMT decoupling caps are recommended around the Neuron Chip on the
component side of the PCB; follow the recommendations in the Neuron Chip
Databooks
2,3
. In contrast to the wide Vcc distribution to the Neuron Chip and other
node circuitry, the routing of Vcc to the FTT-10A transceiver should be through a
narrow trace of 0.3mm (0.012 inch) width. The inductance of this trace in
combination with the FTT-10A transceiver's Vcc decoupling capacitor, C1, helps to
reduce digital noise at the FTT-10A transceiver's Vcc input (pin 1), which reduces
EMI on the network wiring. Place C1 immediately next to the FTT-10A transceiver
as shown in figure 2.3.
7. CLK Routing for EMI Control
: The CLK input to the FTT-10A needs to be guarded
by ground traces to minimize clock noise, and to help keep EMI levels low (see
chapter 5). In general, the
Neuron Chip should be placed close enough to the FTT-
10A transceiver and oriented correctly so that the CLK trace from the Neuron Chip to
the transceiver is no longer than 2cm (0.8"). At the same time, the Neuron
Chip and
any other fast digital circuitry should be kept away from the network connector and
NET_A/NET_B pins (pins 2 and 3) on the transceiver. If noisy digital circuitry is
located too close to the network connector, RF noise may couple out onto the network
cable and cause EMI problems.
8. FTT-10A Transceiver Grounding for EMI Control
: To best control EMI, the
connection between the FTT-10A transceiver ground pin and the center of star
ground should be wide and short as shown in figure 2.3.
Physical Layer Repeater
In the event that the limits on the number of transceivers or total wire distance are
exceeded, a physical layer repeater (figure 2.5) can be added to interconnect two or
more TP/FT-10 network segments. This effectively multiplies overall channel
capability, including node count and network extent, but not bandwidth. To extend
network bandwidth, use L
ONWORKS Routers.
Designing large networks which rely on physical layer repeaters to extend node
count requires careful consideration of network traffic. Testing of the network
under worst-case traffic conditions while monitoring with a LonManager Protocol
Analyzer is highly recommended to properly evaluate system performance.
L
ONWORKS Routers and/or topology changes may be used to solve traffic congestion
problems.