User's Manual

3 -Installing a MiY Device  21
Proprietary
A good rule of thumb is that if the propagation delay of the data line is much less than one bit
width, termination is not needed. This rule makes the assumption that reflections will damp out
in several trips up and down the data line. 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. For example, 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 66–75% of the speed
of light (c), is specified by the cable manufacture.
For our example, a round trip covers 4000 feet of cable. Using a propagation velocity of 0.66 ×
c, one round trip is completed in approximately 6.16 μs. If we assume the reflections will damp
out in three “round trips” up and down the cable length, the signal will stabilize 18.5 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.
There are several methods of terminating data lines. The method recommended by B&B
Electronics is parallel termination. A resistor is added in parallel with the receiver’s “A” and “B”
lines in order to match the data line characteristic impedance specified by the cable
manufacture (120 Ω is a common value)
Termination Resistor
This value describes the intrinsic impedance of the transmission line and is not a function of
the line length. A terminating resistor of less than 90 Ω should not be used. Termination
resistors should be placed only at the extreme ends of the data line, and no more than two
terminations should be placed in any system that does not use repeaters. This type of
termination clearly adds heavy DC loading to a system and may overload port powered RS-232
to RS-485 converters.
Another type of termination, AC coupled termination, adds a small capacitor in series with the
termination resistor to eliminate the DC loading effect. Although this method eliminates DC
loading, capacitor selection is highly dependent on the system properties.
System designers interested in AC termination are encouraged to read National Semiconductors
Application Note 9032 for further information. Refer to the illustration of both parallel and AC
termination on an RS-485 two-wire node. In four-wire systems, the termination is placed across
the receiver of the node.