rs422-rs485-Phan 3 docx

Balanced Line Drivers• In a balanced differential system the voltage produced by the driver appears across a pair of signal lines that transmit only one signal.. It isn’t used by a balan

EIA - Electroonic Industries Association

• The purpose of this note is to describe the

main elements of an RS-422 and RS-485

system.

• This note attempts to cover enough technical details so that the system designer will have considered all the important aspects in the data system design

Balanced Line Drivers

• In a balanced differential system the voltage produced by the driver appears across a pair of signal lines that transmit only one signal (2-6V)

• C: Signal Ground It isn’t used by a balanced line receiver in

determining the logic state of the data line.

• ENABLE: (Tristate condition of the driver)

• RS-485 diver must have ENABLE control signal.

• RS-422 driver may have this signal, but it is not always required.

Balanced Line Receiver

• A balanced differential line receiver sense the voltage state of the transmission line across two signal input lines.

• Signal ground C is necessary in making the proper interface connection

• The 200mV to 6V range is required

to allow for attenuation on the transmission line.

• Common mode voltage is define

as the mean voltage of the A and B terminals with respect to signal ground.

Schmitt Trigger using Op-Amp

vout = A(v2 – v1)

The comparator will switch when V+=0 Then

The effect of using a Schmitt trigger

(B) instead of a comparator (A).

Typical hysteresis curve (Non-inverting)

Non-inverting Schmitt trigger

Inverting Schmitt trigger

The difference between the trip points

is the hysteresis H and is given as

s s

R

R V

R

R V

R

R H

Let's say you detect a low-to-high transition at 2.5 V A

100 mV hysteresis would mean that the low-to-high transition is detected at 2.55 V and the high-to-low transition is detected at 2.45 V, a 100 mV difference.

Vcc = Voltage Common Collector Bipolar Vee = Voltage Common Emitter Bipolar Vss = Voltage Common Source FET Vdd = Voltage Common Drain FET

EIA Standard RS-422 Data Transmission

• Each generator can drive up to 10 receivers.

The 2 signaling states of the line are define as follows:

• When the “A” terminal of the driver is negative with respect to the “B” terminal, the line is in a binary 1 (MARK or OFF) state.

• When the “A” terminal of the driver is positive with respect to the “B” terminal, the line is in a binary 0 (SPACE or ON) state.

• Like RS232, the data bits are transmitted in reverse order.

The terminator is placed at the end of a transmission line or daisy chain bus, designed to match impedance and hence minimize signal reflections.

• Figure shows the condition of the voltage of the balanced line for an RS-232 to RS-422 converter when the line is in the “idle” condition or OFF state.

• The “A” terminal is equivalent to the

• As many as 32 driver/receiver pairs can share a multi drop network

• Many characteristics of the drivers and the receiver are the same as 422.

RS-• The range of the common mode voltage Vcm that the driver and receiver can tolerate is expanded to +12 to -7 volts.

• The driver can be disconnected or tristated from the line.

• The signal ground line is also recommended in an RS-485 system

EIA Standard RS-485 Data Transmission

Tristate control of an RS-485 device using

RTS

Send data control of an RS-485 device

• Termination is used to match impedance of a node of to the impedance of the transmission line being used.

• When impedance are mismatched, the transmitted signal is not

completely absorbed by the load and a portion is reflected back in to the transmission line.

• If the source, transmission line and load impedance are equal these

reflections are eliminated.

• A resistor is added in parallel with the receiver’s “A” and “B” line in order

to match the data line characteristic impedance specified by the cable

manufacture (120 Ω is a common value or greater) A terminating resister

of less than 90 Ω should not be used.

Transient protection of RS-422 and RS-485 system

Isolated RS-485 device

RS-485 Device with signal ground

connected to chassis ground

Signal ground connection between two nodes with 100 Ω resistor

Isolated node with shunt protection to earth ground

Isolated port with ungrounded shunt protection

Surge Transient protection of

RS-422 and RS-485 system

Fused port protection

SN65176B, SN75176B DIFFERENTIAL BUS TRANSCEIVERS

• The SN65176B and SN75176B differential bus

transceivers are integrated circuits designed for

bidirectional data communication on multipoint

bus transmission lines.

• They are designed for balanced transmission

lines and meet ANSI Standards TIA/EIA-422-B

and TIA/EIA-485-A

• The receiver features a minimum input

impedance of 12 kΩ, an input sensitivity of ±200

mV, and a typical input hysteresis of 50 mV.

Full duplex

SN75179B DIFFERENTIAL DRIVER AND

RECEIVER PAIR

The SN75179B is a differential driver and

receiver pair designed for balanced

transmission-line applications and meets

TIA/EIA-422-B, TIA/EIA-485-A It is designed to improve the performance of full-duplex data communications over long bus lines.

Half duplex

CONTROLLING THE DRIVER ENABLE

CONVERT RS232 TO RS485

Half duplex

• A MAX233 converts the RS-232 signals to TTL levels, and

the TTL signals connect to an SN75176B that provides the

RS-485 interface.

• The MAX220–MAX249 family of line drivers/receivers is

intended for all EIA/TIA-232E and V.28/V.24

communications interfaces, particularly applications

where ±12V is not available.

MAX232

MAX233

V.24 is a specification for single-ended communications that includes the definition of

connector pin allocations It is used together with V.28 to define a specification for

serial asynchronous or synchronous communications.

CONVERT RS232 TO RS485 Half duplex

Popular chip MAX232 is used to convert RS232 signals from/to TTL level MAX485 converts TTL level signals to RS485

standard RTS line is used to change transmission mode of MAX485 RTS signal is first converted to TTL level by MAX232 then

connected to DE and RE/ pins 2 LEDs are used to indicate current state of the interface

LM78LXX Series Output voltages of 5.0V, 6.2V, 8.2V, 9.0V, 12V, 15V

The MAX481, MAX483, MAX485, MAX487–MAX491, and MAX1487 are low-power transceivers for RS-485 and RS-422 communication.

CONVERT RS232 TO RS485 Half duplex

This is a small RS232 to RS485 converter project it use for convert RS232 signal level to RS485 level multidrop It can use with 32 slave to communicate with

PC and embedded systems board for long distance less than 1.2Km( 4000 feet).

Specification

- 9-12Vdc power supply

- Small size only 2.8" X 1.45 "

- Use for RS485 multidrop 2 wire connection

- Direct connect with PC on DB9 connecter

- Use RTS signal to control direction

- 32 Slaves (up to 256 slaves with some transceiver i.e MAX3088)

J1 and J2 use for jump R terminate(120) at the end of communication line or last slaver The 75176 use for transceiver.

RTS active (logic 0 or -3 to -15V) when we need to send data

Diode 1N4001 provides

protection against reverse

polarity connection of the

supply Adding a resistor to a line may limit damaging current flows that would

otherwise result from short high-voltage transients, such as those caused by electrostatic discharge (ESD).

Data Format and Protocols

• Information content passing through peer-to-peer

connection is packed in a very simple structure:

– < Header-string > < Code identifier >< INFO-FIELD >< string >

Terminator-– < Header-string > and < Terminator-string > are both configurable via software (device configuration parameters)

• Most common generic Handshake are

available/selectable with RS232 interface:

– Hardware (RTS-CTS)

– Software XON/XOFF