SN65HVD72
SN65HVD75
SN65HVD78
SLLSE11B –MARCH 2012–REVISED JUNE 2012
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APPLICATION INFORMATION
Device Configuration
The SN65HVD72, 'HVD75 and 'HVD78 are half-duplex RS-485 transceivers operating from a single 3.3V ±10%
supply. The driver and receiver enable pins allow for the configuration of different operating modes.
R
R
R
R
R
R
RE
A
B
RE
A
B
RE
A
B
DE
D
DE
D
DE
D
D
D
D
a) Independent driver and
receiver enable signals
b) Combined enable signals for
use as directional control pin
c) Receiver always on
Figure 18. 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 to
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 connected in parallel to a bus cable. To eliminate line
reflections, each cable end is terminated with a termination resistor, RT, whose value matches the characteristic
impedance, Z0, of the cable. This method, known as parallel termination, allows for relatively high data rates over
long cable length.
R
R
R
R
A
B
A
B
RE
RE
R
T
R
T
DE
D
DE
D
D
D
A
B
A
B
R
R
R
R
D
D
D
D
RE DE
RE DE
Figure 19. Typical RS-485 network with SN65HVD7x Transceivers
Common cables used are unshielded twisted pair (UTP), such as low-cost CAT-5 cable with Z0 = 100 Ω, and
RS-485 cable with Z0 = 120 Ω. Typical cable sizes are AWG 22 and AWG 24.
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