User's Manual

Page 24

Ethernet Network Topology

Communication cables for Ethernet logically form a straight line bus, however, the

more devices placed on that bus, the less efficient the network due to increased

collisions and the weaker the signal will get due to increased distance. Repeaters

can be used to boost signal strength, however a better solution would be placing

switches at intermediate positions along the bus. The most common Ethernet

topology in use today is the star configuration (see figure 11), with a hub or switch

in the center.

RS-485 Cable Termination

Most RS-485 buses require termination because of fast transitions, high data rates,

or long cables. The purpose of the termination is to prevent adverse transmission-

line phenomena, such as reflections.

For example, suppose an installation uses 2000 feet of cable. A round trip, then,

covers 4000 feet. Using a propagation velocity of 0.66c (two thirds the speed of

light – contact the cable supplier for this value for your cable), one round trip is

completed in approximately 6.2 micro-seconds. If we assume the reflections will

damp out in three round trips up and down the cable length, the signal will

stabilize 18.6 micro-seconds after the leading edge of a bit. At 56 k-baud (57,600

bits per second) one bit is 17.4 micro-seconds wide. In this case, the reflections do

not damp out before more information is being sent, and corruption of

information is a potential problem. Termination will solve this problem as will

lowering the baud rate. At 9600 baud, the bit width is 104 micro-seconds wide,

the reflections are damped out much before the center of the bit, and

termination is not required. Longer cable length and higher baud rates each push

the case for use of termination.

A common mistake is to connect a terminating resistor at each node - a practice

that causes trouble on buses that have four or more nodes. The active driver sees

the four termination resistors in parallel, a condition that excessively loads the

driver. If each of the four nodes connects a 100Ω termination resistor across the

bus, the active driver sees a load of 25Ω instead of the intended 50Ω. The problem

becomes substantially worse with 32 nodes. If each node includes a 100Ω

termination resistor, the load becomes 3.12Ω.