Renesas RS-485 Quick Start Manual

RS-485: Transmitting Full-Duplex Data over Single Twisted-Pair Cable
The demand for higher data throughput lead to the full-duplex RS-485 interface allowing for the simultaneous
receiving and transmitting of data using two twisted-pair cable
even further, a new type of full-duplex interface has emerged that allows data transmission over a single
twisted-pair cable (Figure
In this new type of full-duplex interface, each transceiver connects to the bus using a 4-to-2 wire converter that
passes the transmit data from the local transceiver onto the bus, while at the same time extracting the receiving
data that is sent by the remote transceiver from the bus.
The interface is purely intended for point-to-point data links, as it requires two, permanently active signal sources.
Therefore, idling a signal source requires it to idle in a bus-high or bus-low state, rather than disabling the driver.
This application note explains the design of the bus node circuitry.
Node 1
Receive
A
R
R
B
RE
DE
Transmit
Z
D D
Y
Figure 1. Traditional Full-Duplex Link with Enable
Controls over Two Twisted-Pair Cables
Contents
1. Bus Node Circuit Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Current Limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Bus Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 4-to-2 Wire Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Deriving the Bus Voltage to Driver Output Voltage Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.5 Deriving the R
1.6 Deriving the R
1.7 Deriving the Driver Output Voltage, V
2. Calculation Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Complete Interface Circuit and Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
R15AN0009EU0100 Rev.1.00
Mar 4, 2022
2).
Node 2
Z
D
Y
A
R
B
/R Ratio of the 4-to-2 Wire Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
B D
and R Resistor Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
B D
(Figure 1). To reduce cable and installation cost
Node 1
A
D
R R
B
DE
RE
Z
D D
Y
R
Figure 2. New permanent active Full-Duplex Link over
OD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Application Note
Receive
P P
N N
Transmit
Single Twisted-Pair Cable
© 2022 Renesas Electronics
Node 2
Z
D D
Y
A
R
R
B
Page 1