Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 29 of 137 Manual Documentation Number: <DocNumber>
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Since the receiving UART will sample the data in the middle of the bit,
it is important that the signal level be solid at that point.
One practical rule of thumb suggests that, for equipment operating
data rates less than 38.4 kbps, termination is usually unnecessary.
Example calculation to determine whether termination is necessary
In a system with 2000 feet of data line the propagation delay can be
calculated by multiplying the cable length by the propagation velocity
of the cable. This value, typically 66 to 75% of the speed of light (c), is
specified by the cable manufacturer. A round trip covers 4000 feet of
cable. Using a propagation velocity of 0.66 × c, one round trip is
completed in approximately 6.2 µs. Assuming the reflections will damp
out in three "round trips" up and down the cable length, the signal will
stabilize 18.6 µs after the leading edge of a bit. At 9600 baud one bit is
104 µs wide. Since the reflections are damped out much before the
center of the bit, termination is not required.
Methods of Termination
There are several methods of terminating data lines. These include
parallel termination, bidirectional termination and AC termination.