Networks that donot need termination
One situation inwhich you don’t need termination networks is when the two-way loop time of thenetwork is much greater than a single bit time (~<0.1×two-way loop delay).In such scenarios, the reflections will lose energy each time they reach an endof the network.
As you can seefrom Figure 1, the amplitude of the reflections will continue to decay eachtime the signal reflects at the end of the cable. Figure 1 shows three roundtrips for the signal and a total of six reflections.
Figure 1:Amplitude of reflection decay each time a reflection occurs
Estimating thatthe unterminated end of the bus has a 96kΩ input impedance (a one-eighth unitload), and the source impedance of the driver is 60Ω, the signal reflectionswould decay according to the calculations listed in Table 1.
Table 1:Example signal-decay calculations
As Table 1shows, by the time the signal reflects for the sixth time, it has decayed tounder 4% of its original magnitude. After this point it is safe to say that thereflections are no longer capable of causing signal-integrity issues. Since thesample point of a bit typically occurs between 50-75% of the way through thebit, you would want to make sure that these three round-trip delays occurbefore the sample point.2 y& W/ c( [/ H6 L- z, I$ }4 p