Datasheet

LTC2870/LTC2871
24
28701fb
For more information www.linear.com/LTC2870
applicaTions inForMaTion
RS485 Receiver Input Resistance
The RS485 receiver input resistance from A or B to GND
(Y or Z to GND in half-duplex mode with driver disabled)
is greater than 96kΩ (typically 125kΩ) when the inte-
grated termination is disabled. This permits up to a total
of 256 receivers per system without exceeding the RS485
receiver loading specification. The input resistance of the
receiver is unaffected by enabling/disabling the receiver
or whether the part is in half-duplex, full-duplex, loopback
mode, or even unpowered. The equivalent input resis-
tance looking into the RS485 receiver pins is shown in
Figure18.
Figure18. Equivalent RS485 Receiver
Input Resistance Into A and B (Note 5)
28701 F17
A
B
TE485
60Ω
60Ω
125k
125k
When the TE485 pin is high, the termination resistors are
enabled and the differential resistance from A to B and Y
to Z is 120Ω. The resistance is maintained over the entire
RS485 common mode range of –7V to 12V as shown in
Figure19.
RS485 Half- and Full-Duplex Control
The LTC2870 and LTC2871 are equipped with a control to
switch between half- and full-duplex operation. With the
H/F pin set to a logic low, the A and B pins serve as the
differential receiver inputs. With the H/F pin set to a logic
high, the Y and Z pins serve as the differential inputs. In
either configuration, the RS485 driver outputs are always
on Y and Z. The impedance looking into the A and B pins
is not affected by H/F control, including the differential
termination resistance. The H/F control does not affect
RS232 operation.
RS485 Polarity and the LTC1334, LTC1387
The LTC2870/LTC2871 receiver uses A as the positive
input and B as the negative input. The receiver output is
a logic high when A−B is positive. For the RS485 driver,
a high input produces a positive differential voltage on
Y–Z. In contrast, the LTC1334 and LTC1387 dual proto-
col devices use the opposite RS485 convention: A is the
negative receiver input, B is the positive receiver input, Y
is the negative driver output, and Z is the positive driver
output.
VOLTAGE (V)
–10
RESISTANCE (Ω)
126
124
122
118
120
116
10–5
28701 F18
1550
V
CC
= 5.0V
V
CC
= 3.3V
Figure19. Typical Resistance of the Enabled RS485
Terminator vs Common Mode Voltage on A /B
Selectable RS485 Termination
Proper cable termination is important for good signal
fidelity. When the cable is not terminated with its charac-
teristic impedance, reflections cause waveform distortion.
The LTC2870 and LTC2871 offer integrated switchable
120Ω termination resistors between the differential
receiver inputs and also between the differential driver
outputs. This provides the advantage of being able to eas-
ily change, through logic control, the proper line termina-
tion for correct operation when configuring transceiver
networks. Termination should be enabled on transceivers
positioned at both ends of the network bus. Termination
on the driver nodes is important for cases where the driver
is disabled but there is communication on the connecting
bus from another node. Differential termination resistors
are never enabled in RS232 mode on the LTC2870.
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.