Datasheet
R
T
R
T
R
A B
R RE DE D
D
R
A B
R RE DE D
D
R
D
R
RE
DE
D
A
B
R
D
R
RE
DE
D
A
B
R
D
R
RE
DE
D
A
B
R
D
R
RE
DE
D
A
B
R
D
R
RE
DE
D
A
B
a) Independent driver and
receiver enable signals
b) Combined enable signals for
use as directional control pin
c) Receiver always on
SN65HVD82
www.ti.com
SLLSED6 –OCTOBER 2012
APPLICATION INFORMATION
DEVICE CONFIGURATION
The SN65HVD82 is a half-duplex, 250 kbps, RS-485 transceiver operating from a single 5V supply. The driver
and receiver enable pins allow for the configuration of different operating modes.
Figure 13. SN65HVD82 Transceiver Configurations
Using independent enable lines provides the most flexible control as it allows for the driver and the receiver to be
turned on and off individually. While this configuration requires two control lines, it allows for selective listening
into the bus traffic, whether the driver is transmitting data or not.
Combining the enable signals simplifies the interface to the controller by forming a single, direction-control signal.
Thus, when the direction- control line is high, the transceiver is configured as a driver, while for a low the device
operates as a receiver.
Tying the receiver-enable to ground and controlling only the driver-enable input, also uses one control line only.
In this configuration a node not only receives the data from the bus but also the data it sends and thus can verify
that the correct data have been transmitted.
BUS – DESIGN
An RS-485 bus consists of multiple transceivers connecting in parallel to a bus cable. To eliminate line
reflections, each cable end is terminated with a termination resistor, R
T
, whose value matches the characteristic
impedance, Z
0
, of the cable. This method, known as parallel termination, allows for higher data rates over longer
cable length.
Figure 14. Typical RS-485 Network with SN65HVD82 Transceivers
Common cables used are unshielded twisted pair (UTP), such as low-cost CAT-5 cable with Z
0
= 100 Ω, and
proper RS-485 cable with Z
0
= 120 Ω.
Line measurements have shown that making R
T
by up to 10% larger than Z
0
improves signal quality. Typical
cable sizes are AWG 22 and AWG 24.
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