LTC491
APPLICATI
S I FOR ATIO
U
When using low loss cables, Figure 12 can be used as a
guideline for choosing the maximum line length for a given
data rate. With lower quality PVC cables, the dielectric loss
factor can be 1000 times worse. PVC twisted pairs have
terrible losses at high data rates (>100kBs), and greatly
reduce the maximum cable length. At low data rates
however, they are acceptable and much more economical.
Cable Termination
The proper termination of the cable is very important.
If the cable is not terminated with it’s characteristic
impedance, distorted waveforms will result. In severe
cases, distorted (false) data and nulls will occur. A quick
look at the output of the driver will tell how well the cable
is terminated. It is best to look at a driver connected to the
end of the cable, since this eliminates the possibility of
getting reflections from two directions. Simply look at the
driver output while transmitting square wave data. If the
cable is terminated properly, the waveform will look like a
square wave (Figure 13).
If the cable is loaded excessively (47Ω), the signal initially
sees the surge impedance of the cable and jumps to an
initial amplitude. The signal travels down the cable and is
reflected back out of phase because of the mistermination.
0.1
1.0
10
100
PROBE HERE
LTC491 • F11
Cables and Data Rate
The transmission line of choice for RS485 applications is
a twisted pair. There are coaxial cables (twinaxial) made
for this purpose that contain straight pairs, but these are
less flexible, more bulky, and more costly than twisted
pairs. Many cable manufacturers offer a broad range of
120Ω cables designed for RS485 applications.
Losses in a transmission line are a complex combination
of DC conductor loss, AC losses (skin effect), leakage and
AC losses in the dielectric. In good polyethylene cables
such as the Belden 9841, the conductor losses and
dielectric losses are of the same order of magnitude,
leading to relatively low over all loss (Figure 11).
10
LOSS PER 100 ft (dB)
1.0
0.1
FREQUENCY (MH
Z
)
Figure 11. Attenuation vs Frequency for Belden 9481
DX
DRIVER
10k
Rt = 120Ω
CABLE LENGTH (ft)
1k
100
10
10k
100k
1M
2.5M
DATA RATE (bps)
LTC491 • F12
Figure 12. Cable Length vs Data Rate
8
W
U
UO
Rt
RECEIVER
RX
Rt = 47Ω
Rt = 470Ω
10M
LTC491 • F13
Figure 13. Termination Effects
491fa
LINER [ LINEAR TECHNOLOGY ]
