Copyright © 2017, Assoc.Prof.Dr.

Vo Tuong Quan

INTRODUCTION TO RS – 485 STANDARD

OVERVIEW -

RS-485 is a solution for applications that need to communicate

over longer distances or at higher speeds than RS-232 can

handle. RS-485 also isn’t limited to two devices. An RS-485

network can connect as many as 256 computers or

devices/Nodes along a single pair of wires.

RS-485 has several advantages over RS-232:

 Low cost. The drivers and receivers are inexpensive and

require just a single +5V (or lower) supply to generate the

required minimum 1.5V difference at the differential outputs.

In contrast, RS-232’s minimum output swing of ±5V requires

dual supplies or expensive interface chips to generate the

voltages.

 Networking ability. Instead of being limited to two devices,

RS-485 is a multidrop interface that can have multiple drivers

and receivers. With high-impedance receivers, an RS-485

network can have up to 256 nodes.

 Long links. An RS-485 cable can be as long as 4000 ft

(1219m) compared to RS-232’s typical limit of 50 to 130 ft.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

 Speed. The bit rate can be as high as 10 Mbps.

 The cable length and bit rate are related. Lower bit rates

allow longer cables.

SPECIFICATION

The main reason why RS-485 can transmit over long distances

is its use of balanced lines, which have excellent noise

immunity. Each signal has a dedicated pair of wires. The

voltage on one wire equals the negative, or complement, of the

voltage on the other wire. The receiver detects the difference

between the voltages.

VOLTAGE REQUIREMENT

RS-485 interfaces typically use a single power supply of 3.3V or

5V, but the logic levels at the drivers and receivers differ from

the voltages used by 3.3/5V TTL/CMOS logic chips. For a valid

output, the difference between outputs A and B must be at least

1.5V.

SPEED OF COMMUNICATION

An RS-485 line can have a bit rate as fast as 10 Mbps or as

long as 4000 ft (about 1219m) but not both at the same time.

Longer cables require slower bit rates because the cable’s

capacitance slows the signal transitions.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

Fig. 1 RS485 supports up to 10Mbps but the higher bit rate

require shorter cable.

BALANCE AND UNBALANCE LINE

The main reason why RS-485 can transmit over long distances

is its use of balanced lines, which have excellent noise

immunity. Each signal has a dedicated pair of wires. The

voltage on one wire equals the negative, or complement, of the

voltage on the other wire. The receiver detects the difference

between the voltages. TIA-485-A designates the two lines in a

differential pair as A and B.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

At a typical RS-485 driver, a TTL logic-high input brings line A

more positive than line B, while a TTL logic-low input brings

line B more positive than line A. At the RS-485 receiver, if input

A is more positive than input B, the TTL output is logic high,

and if input B is more positive than input A, the TTL output is

logic low.

Referenced to the receiver’s ground, each input must be within

the range -7V to +12V. This range allows for differences in

ground potential between the driver and receiver. The

maximum allowed differential voltage, or the difference

between the voltages on line A and line B, is ±6V.

Balanced lines are quiet because the two signal wires carry

nearly equal but opposite currents. The currents reduce

received noise because most noise is present more or less

equally on both wires, and any noise voltage that shows up on

one line is canceled by an identical voltage on the other. The

source of noise can be signals on other wires in the cable or

signals that couple into the wires from outside the cable. A

balanced receiver sees only the transmitted signal with noise

eliminated or very much reduced.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

In contrast, in an unbalanced interface, the receiver detects the

voltage difference between the signal wire and ground. When

multiple signals share a ground wire, each of the return

currents induces voltages on the ground shared by all.

Parallel interfaces can have eight or more lines switching

constantly, and even serial interfaces often have two data lines

and multiple status and control signals.

If the ground connects to an earth ground, noise from other

sources can affect the circuits as well.

Another advantage to balanced lines is that they are immune,

within limits, to differences in ground potential between the

driver and receiver. In a long cable, the grounds at the driver

and receiver may vary by many volts. On an unbalanced

line, ground differences can cause a receiver to misread an

input. A balanced line can ignore mismatched grounds (up to a

limit) because the receiver is detecting the difference between

the two transmitted signals.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

Fig. 2 An unbalanced line uses one wire per signal while a

balanced line uses two wires per signal. Both types of line must

have a common ground reference

INTERFACING

Interface chips are available to convert between TTL/CMOS and

RS-485 logic levels. RS-232 interfaces can also be converted to

RS-485.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

SN75176B is a chip that contains a single RS-485 transceiver. A

driver converts a TTL/CMOS voltage at DI to an RS-485

differential voltage, and a receiver converts an RS-485

differential voltage to a TTL/CMOS-compatible voltage at RO.

The driver and receiver each have an enable input. The Texas

Instruments SN75176B is an early chip that used this

configuration and pinout.

CONVERTING VOLTAGE

The asynchronous serial port on a microcontroller can use 3.3V

CMOS or 5V TTL/CMOS logic levels. There are several ways to

convert between 3.3/5V logic and RS-485.

Full duplex

Most RS-485 lines are half-duplex, where multiple drivers and

receivers share a signal path. But you can also use RS-485 in a

full-duplex line, where each direction has its own signal path.

As long as you include any required flow-control signals in the

interface, you can swap an RS-232 line for a full-duplex RS-

485 with no changes to the software or firmware that uses the

interface. Both can use the same programming, though RS-485

supports higher bit rates and the hardware allows longer cables.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

For a full-duplex line, you can use the Texas Instruments

SN75179B or similar chips. The SN75179B contains a driver

that translates 5V TTL signals to RS-485 and a receiver that

translates RS-485 back to 5V TTL.

This chip is a solution when you want to create a long-distance,

full-duplex link between microcontrollers. The RS-485

interface chips are smaller and cheaper than RS-232 interface

chips. Example in Fig. 3.

Fig. 3 A full-duplex RS-485 circuit provides a data path for

each direction

Figure 4 below shows how to use multiple drivers and receivers

in full-duplex communications. One arrangement is in a

primary/secondary network, where a primary computer (Node 0

in the figure) has control of the network and grants the

secondary computers (Nodes 1 and higher) permission to

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

transmit. One pair of wires connects Node 0’s driver to all of

the secondary nodes’ receivers. In the other direction, another

pair of wires connects all of the secondary nodes’ drivers to

Node 0’s receiver. All of the secondary nodes monitor

messages from Node 0. The node being addressed replies on

the other pair of wires. The advantage is that the secondary

nodes don’t see the other nodes’ replies. With a single data

path, all of the secondary nodes receive all of the network

traffic.

Fig. 4 In this full-duplex, multi-node circuit, Node 0 transmits

Many RS-485 circuits are half-duplex, where multiple drivers

and receivers share a signal path. The interfaces form a serial

network, and each computer with an RS-485 interface is a node

in the network.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

Networks typically use half duplex interfaces and allow one

node at a time to transmit. Links with just two devices can be

half duplex as well. Microcontrollers that allow configuring a

port bit as input or output can send and receive on a single

port bit, reconfiguring the bit as needed. You might use this

approach if you need to use the fewest number of port pins or

wires possible. If you need to transmit in just one direction

(simplex), you of course need only one data path.

Figure 5 shows a half-duplex network that uses an SN75176B

differential bus transceiver.

When a driver’s enable input (DE) is low, the driver’s output is

high impedance, and for all practical purposes the driver is

removed from the circuit. When a receiver’s enable input (/RE)

is high, the receiver’s output is high impedance and no longer

follows the RS-485 input.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

Fig. 5 This half-duplex RS-485 network has a single data path.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

HOW TO CONVERT RS232 TO RS485

If a computer has an available RS-232 port, adding an external

converter can be feasible.

Note: RS-232 drivers can limit the top speed of the RS-485

interface.

Microcontroller development boards sometimes have RS-232

interfaces built-in. If you need RS-485, it might be easier to

bypass the RS-232 interface by removing the RS-232 interface

chip or the connections to it and wiring the RS-485 interface

directly to the microcontroller’s port pins.

Half duplex

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

Fig. 6 converts between RS-232 and TTL, and between TTL and
RS-485.

Figure 6 shows one way to convert RS-232 to RS-485. The

interface uses three RS-232 lines: TX transmits data, RX

receives data, and RTS controls direction. A MAX233 converts

the RS-232 signals to TTL levels, and the TTL signals connect

to an SN75176B or similar chip that provides the RS-485

interface. When RTS is low, the enable inputs of the SN75176B

are high and TX can transmit on the RS-485 line. When RTS is

high, the enable inputs are low and RX can receive data on the

RS-485 line. A circuit can use DTR instead of RTS.

In a similar way, you can create full-duplex RS-232-to-RS-485

interfaces using a SN75179B or other 4-wire RS-485 chip.

Full Duplex

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

Fig. 7 link between a device with an RS-485 interface

Figure 7 shows a connection between an RS-232 port and an

RS-485 port. RS-485’s B output connects to RS-232’s RX input.

Referenced to signal ground, the B output is near 0V for a logic

1 and near +5V for a logic 0.

CONTROLLING THE DRIVER ENABLE

In an RS-485 network, only one driver can be enabled at a time.

After sending data, a node should disable its driver as quickly

as possible so the next node to transmit can enable its driver

and start transmitting.

Software Based Control

Fig. 8 Three options for controlling the driver-enable and

receiver-enable inputs on an RS-485 transceiver.

Controlling the driver in software requires a dedicated output

bit that connects to the transceiver’s driver-enable input. On a

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

PC’s RS-485 port that uses an RS-232 converter or USB/serial

converter, RTS or DTR can serve as the driver-enable output.

Other ports can use any spare output bit.

An RS-485 port can use any of three configurations (Figure 8):

• Use two output bits and control the driver’s and receiver’s

enable inputs separately. This arrangement provides the most

flexible control but requires two port bits.

• Use one output to control both enable lines. The driver’s

enable input is active-high and the receiver’s enable input is

active-low so either the driver or receiver is always enabled and

both are never enabled at the same time.

This configuration is useful if a node doesn’t need to receive its

own transmissions.

• Control the driver-enable input only. Tie the receiver’s enable

input low to keep the receiver enabled at all times. With this

configuration, a node receives the data it sends and thus can

verify that the data transmitted.

Hardware Based Control (Self Study)

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

Example

Design the RS232 – RS485 converter

Fig. 9 RS232 – RS485 Interface

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

Fig. 9 PC – RS485 Interface

U3,U1 H11L1 OPTO-ISOLATOR

U2 CNY17-3 OPTO-ISOLATOR

U4 MAX487, SN75176B

Figure 9 shows the circuit diagram of RS485 interface.

Connector K1 is linked to the serial port of the PC, power to the

PC side of the circuit is derived from the signal lines DTR and

RTS. Positive supply is derived from RTS and negative supply

from the DTR line. The RTS line is also used to control the data

direction of RS485 driver IC U4.

Optical isolation is achieved by optocouplers U1, U2 and U3.

Opto U1 is used to control the data direction of U4 opto U2

provide RXD line isolation while opto U3 provide TXD line

isolation.

The other side of the isolator carries TTL levels. This side is

powered by an unregulated dc supply between 9V and 18V dc.

IC U5 provide 5V regulated output and IC U4 provide the RS485

bus interface. The TXD and RXD lines status are provided by

data indicating LEDs.

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 Copyright © 2017, Assoc.Prof.Dr. Vo Tuong Quan

For Data Reception RTS = 1 (at +ve level)

For Data Transmition RTS = 0 (at -ve level)

DTR line is always set to 0 (at -ve level)

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