The voltage thus doubles and brings the voltage and current to
their final values after just one reflection.
This type of termination can be useful in full-duplex lines between a single
driver and receiver. The termination uses much less current than a parallel termination.
This termination isn??™t recommended for networks with multiple
nodes because the nodes at different locations on the line will see different
reflections.
1 3 ! %
If the calculations show that a line is electrically short, you might need no
added termination at all. But on some short lines with fast rise times, the components
form a resonant circuit that results in ringing voltages when an output
switches. In these cases, a termination can ensure good signal quality at the
receiver.
Figure 7-7: A series termination can absorb reflections.
Designing RS-485 Links and Networks
123
The amplitude of the ringing varies with the driver??™s output resistance, the
wires??™ inductance, the load??™s capacitance, and the frequencies carried by the
wires. As with other mismatched terminations, if the ringing voltages are large
enough, the receiver may misread transmitted bits.
To reduce ringing, use a driver with a slower rise time. There??™s no reason to use
a driver capable of 10 Mbps if you??™re transmitting at 9600 bps. If you can??™t
change the hardware, using a slow bit rate at least gives the ringing more time to
settle before the receiver reads the input.
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