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LTC491CS#PBF

P1-P3P4-P6P7-P9P10-P12
7
LTC491
491fa
Typical Application
A typical connection of the LTC491 is shown in Figure 9.
Two twisted-pair wires connect up to 32 driver/receiver
pairs for full duplex data transmission. There are no
restrictions on where the chips are connected to the wires,
and it isn’t necessary to have the chips connected at the
ends. However, the wires must be terminated only at the
ends with a resistor equal to their characteristic imped-
ance, typically 120Ω. The input impedance of a receiver is
typically 20kΩ to GND, or 0.6 unit RS-485 load, so in
practice 50 to 60 transceivers can be connected to the
same wires. The optional shields around the twisted pair
help reduce unwanted noise, and are connected to GND at
one end.
The LTC491 can also be used as a line repeater as shown
in Figure 10. If the cable length is longer than 4000 feet, the
LTC491 is inserted in the middle of the cable with the
receiver output connected back to the driver input.
Thermal Shutdown
The LTC491 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°C and turns them back on when the temperature cools
to 130°C. If the outputs of two or more LTC491 drivers are
shorted directly, the driver outputs can not supply enough
current to activate the thermal shutdown. Thus, the ther-
mal shutdown circuit will not prevent contention faults
when two drivers are active on the bus at the same time.
APPLICATIO S I FOR ATIO
WUUU
Figure 9. Typical Connection
Figure 10. Line Repeater
LTC491 • F09
120Ω
120Ω
LTC491
DRIVER
RECEIVER
LTC491
DRIVER
DX
RX
DX
RX
2
5
9
10
11
12
RECEIVER
9
10
11
12
5
2
120Ω
3
4
3
4
120Ω
RECEIVER
LTC491
DRIVER
9 10 11 12
5
4
3
2
DX RX
LTC491 • F10
120Ω
LTC491
DRIVERDX
RX
2
5
9
10
11
12
RECEIVER DATA IN
DATA OUT
3
4
120Ω
LTC491
8
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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).
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.
FREQUENCY (MH
Z
)
0.1
0.1
LOSS PER 100 ft (dB)
1.0
10
1.0 10 100
LTC491 • F11
DATA RATE (bps)
10k
10
CABLE LENGTH (ft)
100
1k
10k
100k 1M 10M
LTC491 • F12
2.5M
Figure 12. Cable Length vs Data Rate
Figure 11. Attenuation vs Frequency for Belden 9481
Figure 13. Termination Effects
Rt
DRIVERDX RECEIVER RX
Rt = 120Ω
Rt = 47Ω
Rt = 470Ω
LTC491 • F13
PROBE HERE
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LTC491
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When the reflected signal returns to the driver, the ampli-
tude will be lowered. The width of the pedestal is equal to
twice the electrical length of the cable (about 1.5ns/foot).
If the cable is lightly loaded (470Ω), the signal reflects in
phase and increases the amplitude at the driver output. An
input frequency of 30kHz is adequate for tests out to 4000
feet of cable.
AC Cable Termination
Cable termination resistors are necessary to prevent un-
wanted reflections, but they consume power. The typical
differential output voltage of the driver is 2V when the
cable is terminated with two 120Ω resistors, causing
33mA of DC current to flow in the cable when no data is
being sent. This DC current is about 60 times greater than
the supply current of the LTC491. One way to eliminate the
unwanted current is by AC coupling the termination resis-
tors as shown in Figure 14.
The coupling capacitor must allow high-frequency energy
to flow to the termination, but block DC and low frequen-
cies. The dividing line between high and low frequency
depends on the length of the cable. The coupling capacitor
must pass frequencies above the point where the line
represents an electrical one-tenth wavelength. The value
of the coupling capacitor should therefore be set at 16.3pF
per foot of cable length for 120Ω cables. With the coupling
capacitors in place, power is consumed only on the signal
edges, and not when the driver output is idling at a 1 or 0
state. A 100nF capacitor is adequate for lines up to 4000
feet in length. Be aware that the power savings start to
decrease once the data rate surpasses 1/(120Ω × C).
Receiver Open-Circuit Fail-Safe
Some data encoding schemes require that the output of
the receiver maintains a known state (usually a logic 1)
when the data is finished transmitting and all drivers on the
line are forced into three-state. The receiver of the LTC491
has a fail-safe feature which guarantees the output to be in
a logic 1 state when the receiver inputs are left floating
(open-circuit). However, when the cable is terminated with
120Ω, the differential inputs to the receiver are shorted
together, not left floating. Because the receiver has about
70mV of hysteresis, the receiver output will tend to main-
tain the last data bit received, but this is not guaranteed.
The termination resistors are used to generate a DC bias
which forces the receiver output to a known state; in the
case of Figure 15, a logic 0. The first method consumes
about 208mW and the second about 8mW. The lowest
power solution is to use an AC termination with a pull-up
resistor. Simply swap the receiver inputs for data proto-
cols ending in logic␣ 1.
LTC491 • F14
120Ω
RECEIVER
RX
C
C = LINE LENGTH (ft) x 16.3pF
Figure 14. AC Coupled Termination
Figure 15. Forcing “O” When All Drivers are Off
140Ω
RECEIVER
RX
5V
1.5kΩ
RECEIVER
RX
5V
110Ω
130Ω110Ω 130Ω
LTC491 • F15
120Ω
RECEIVER
RX
C
5V
100kΩ
1.5k
P1-P3P4-P6P7-P9P10-P12

LTC491CS#PBF

Mfr. #:
Buy LTC491CS#PBF
Manufacturer:
Analog Devices Inc.
Description:
RS-422/RS-485 Interface IC Low Power RS485 Tx/Rx
Lifecycle:
New from this manufacturer.
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