User's Manual

Models CH45
EIA 422 or 485
3-3
FIGURE 13. Number of Repeats
3.3.1 Effects of Data Rate
The number of repeaters is an inverse linear function to the data rate (more repeats at lower
data rates). The data rate, or bits per second rate, determines what the original pulse width of
each bit will be. The higher the data rate, the smaller the pulse width of each bit. As the signal
passes through a repeater, any distortion effects on the data signal are greater at higher data
rates due to smaller pulse widths than lower data rates.
3.3.2 Pulse Width Distortion
As the data signal is passed from repeater to repeater, there is a small change to the pulse width
defined as pulse width distortion. The amount of change that is tolerable corresponds to the
percentage of original pulse width required by a particular communication system design.
Typically, a communication system requires the data word, or bit stream, that each Slave IED
receives, match the signal originally generated by the Master, within some tolerance for pulse
width distortion. High tolerance systems allow more pulse width distortion, therefore, more
repeats are tolerated. Conversely, low tolerance systems allow fewer repeats in the loop.
Figure 13 shows the maximum number of repeats possible if 70% of the original pulse width is
required by any IED in the loop. The acceptable percentage of the original pulse width is due to
the requirements of the IEDs. If more of the original pulse width is necessary or less is allowable,
then the number found in Figure 13 can be modified. Table 2 shows the factors to be used to
correct the number of repeats found in Figure 13 for such cases.
Number of Repeats in a Loop Configuration*
1
10
100
1000
1 10 100 1000
Data Rates (kbps)
Repeats
* 65
o
C Amb. – 70% of original pulse width