terminated only at the receiving end. The optional shields
are connected to GND at one end.
in Figure 6. If the cable length is longer than 4000 feet, the
receiver output connected back to the driver input.
120鈩?/div>
7
DATA IN
LOSS PER 100 FT (dB)
6
DX
3
DRIVER
5
DATA OUT
LTC490 鈥?TA07
Figure 6. Line Repeater
Thermal Shutdown
The LTC490 has a thermal shutdown feature which pro-
tects the part from excessive power dissipation. If the
outputs of the driver are accidently shorted to a power
supply or low impedance, source, up to 250mA can flow
through the part. The thermal shutdown circuit disables
the driver outputs when the internal temperature reaches
150擄C and turns them back on when the temperature
cools to 130擄C. If the outputs of two or more LTC490
drivers are shorted directly, the driver outputs can not
supply enough current to activate the thermal shutdown.
Thus, the thermal shutdown circuit will not prevent con-
tention faults when two drivers are active on the bus at the
same time.
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
120鈩?cables designed for RS485 applications.
CABLE LENGTH (FT)
6
U
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 overall loss (Figure 7).
10
1.0
0.1
0.1
1.0
10
100
LTC490 鈥?TA08
W
U
UO
FREQUENCY (MHz)
Figure 7. Attenuation vs Frequency for Belden 9841
When using low loss cables, Figure 8 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.
10k
1k
100
10
10k
100k
1M
2.5M
10M
DATA RATE (bps)
LTC490 鈥?TA09
Figure 8. RS485 Cable Length Specification. Applies for 24
Gauge, Polyethylene Dielectric Twisted Pair.
1
2
3
4
5
6
7
8
