01 RS232 Commm

Serial Communication

(Recommended Standard-232)

J. arumugam

C & I Lab

Communication Principle

Every data communications system requires:

� A source of data (a transmitter or line driver), which

converts the information into a form suitable for

transmission over a link

� A receiver that accepts the signal and converts it back

into the original data

� A communications link that transports the signals. This

can be copper wire, optical fiber, and radio or satellite link

j arumugam PC - 100 2

Communication factors

� The type of signaling used.

� Defining a logical ‘1’ and a logical ‘0’.

� The codes that represent the symbols.

� Maintaining synchronization between transmitter and receiver.

� How the flow of data is controlled, so that the receiver is not swamped.

� How to detect and correct transmission errors.

– The physical factors are referred to as the ‘interface standard’; the

other factors comprise the ‘protocols’

– A protocol is one or a few sets of hardware and software rules

agreed to by all communication parties for exchanging data correctly

and efficiently.


Communication Modes

�Simplex

�Half duplex

�Full duplex

A simplex system is one that is designed for sending messages


A simplex system is one that is designed for sending messages

in one direction only.

Communication Modes cont�

� Half duplex occurs when data can flow in both directions, but in only

one direction at a time

� Full duplex system is designed for sending messages in both

directions simultaneously.


Protocols


Basic structure of an information frame defined by a protocol

RS 232 interface standard

� The RS-232 interface standard specifies the method of connection

of two devices DTE and DCE.

DTE: Data Terminal Equipment.

Example: Computer or Printer.

- DTE device communicates with a DCE device.


DCE: Data Communications Equipment.

- Example: Modem, now also called Data Circuit-Terminating

equipment in RS-232E.

- A DCE device receives data from the DTE and retransmits via

another data communications link, such as the telephone system.

Typical serial data communications link


DTE and DCE

DTE normally comes with a Male Connector, while DCE comes with a

Female Connector. (that is not always true).

Use the simple way below to confirm:

Measure Pin 3 (TxD) and Pin 5 (GND) of a DB-9 Connector with a Volt

Meter, if you get a voltage of -3V to -15V, then it is a DTE device.


Meter, if you get a voltage of -3V to -15V, then it is a DTE device.

If the voltage is on Pin 2 (TxD) and Pin 5 (GND) of a DB-9 Connector

with a Volt Meter, if you get a voltage of -3V to -15V, then it is a DCE

device.

Note: The result for a DB-25 Connector is reversed

RS 232 Electrical signal characteristics

The RS-232 Transmitter is required to produce voltages in the range:

• Logic 1: –5 V to –25 V

• Logic 0: +5 V to +25 V

• Undefined logic level: +5 V to –5 V

• .

At the RS-232 Receiver the following voltage levels are defined:

• Logic 1: –3 V to –25 V

• Logic 0: +3 V to +25 V

• Undefined logic level: –3 V to +3 V

– Note: The RS-232 transmitter requires the slightly higher voltageto overcome voltage drop along the line.


RS-232 Transmitters and Receivers


Pin Functions for RS-232

DTE Pin Assignment (DB-9) DCE Pin Assignment (DB-9)

1 DCD Data Carrier Detect 1 DCD Data Carrier Detect

2 RxD Receive Data 2 TxD Transmit Data


2 RxD Receive Data 2 TxD Transmit Data

3 TxD Transmit Data 3 RxD Receive Data

4 DTR Data Terminal Ready 4 DSR Data Set Ready

5 GND Ground (Signal) 5 GND Ground (Signal)

6 DSR Data Set Ready 6 DTR Data Terminal Ready

7 RTS Request to Send 7 CTS Clear to Send

8 CTS Clear to Send 8 RTS Request to Send

9 RI Ring Indicator 9 RI Ring Indicator

Pin Functions for RS-232

Data Typical purpose

RXD (pin 2) Serial Data Input Carries data from DCE to DTE

TXD (pin 3) Serial Data Output Carries data from DTE to DCE


TXD (pin 3) Serial Data Output Carries data from DTE to DCE

Handshake

RTS (pin 7) Request to Send DTE requests the DCE prepare to receive data

CTS (pin 8) Clear to Send Indicates DCE is ready to accept data

DSR (pin 6) Data Set Ready DCE is ready to receive commands or data

DCD (pin 1) Data Carrier Detect DCE is connected to the telephone line

DTR (pin 4) Data Terminal Ready Indicates presence of DTE to DCE

Ground

GND (pin 5) Ground Common Ground

Other

RI (pin 9) Ring Indicator DCE has detected an incoming ring signal on the telephone line

Null-Modem connection

Using for DTE–DTE, DCE–DCE Devise Communication


Straight / Null-Modem connection

Straight-through (DB-9) Crossover (Null-Modem) (DB-9)

Using for DTE - DCE Devise

Communication

Using for DTE–DTE, DCE–DCE

Devise Communication

(DTE) (DCE) (DTE) (DTE)

1 DCD ------- DCD 1 1 DCD DCD 1

2 RxD ------- TxD 2 2 RxD ------- TxD 3

3 TxD ------- RxD 3 3 TxD ------- RxD 2


3 TxD ------- RxD 3 3 TxD ------- RxD 2

4 DTR ------- DSR 4 4 DTR ------- DSR 6

5 GND ------- GND 5 5 GND ------- GND 5

6 DSR ------- DTR 6 6 DSR ------- DTR 4

7 RTS ------- CTS 7 7 RTS ------- CTS 8

8 CTS ------- RTS 8 8 CTS ------- RTS 7

9 RI ------- RI 9 9 RI RI 9

Transmission characteristics

Signaling Rate (Baud Rate)

� The signaling rate of a communications link is a measure of how

many times the physical signal changes per second and is

expressed as the baud rate. An oscilloscope trace of the data

transfer would show pulses at the baud rate. For a 1000 baud rate,transfer would show pulses at the baud rate. For a 1000 baud rate,

pulses would be seen at multiples of 1 ms.

� With asynchronous systems, we set the baud rate at both ends of

the link so that each physical pulse has the same duration.


Transmission characteristics

Data rate or Bit rate

� The data rate or bit rate is expressed in bits per second (bps), or

multiples such as Kbps, Mbps and Gbps (kilo, mega and gigabits

per second). This represents the actual number of data bits

transferred per second.

� An example is a 1000 baud RS-232 link transferring a frame of 10� An example is a 1000 baud RS-232 link transferring a frame of 10

bits, being 7 data bits plus a start, stop and parity bit. Here the Baud

Rate is 1000 baud, but the Data Rate is 700 bps.

� Baud rate and Bit rate, they are not the same. Whereas baud rate

indicates the number of signal changes per second, the bit rate

indicates the number of bits represented by each signal change.


Transmission characteristics cont�

Bandwidth

� The single most important factor that limits communication speeds is

the bandwidth of the link. Bandwidth is generally expressed in hertz

(Hz), meaning cycles per second.

� Bandwidth is closely related to the transmission medium, ranging� Bandwidth is closely related to the transmission medium, ranging

from around 5000 Hz for the public telephone system to the GHz

range for optical fiber cable.

� As a signal tends to attenuate over distance, communications links

may require repeaters placed at intervals along the link, to boost the

signal level.


Serial Data Communication Methods

�Asynchronous has no clock, but speed must be

agreed upon beforehand (baud rate).

Asynchronous System Synchronous System

�Synchronous communications requires clock.

Whoever controls the clock controls

communication speed.


Asynchronous System

� Asynchronous Communication has no timing signal or clock.

� Instead, it inserts Start/Stop bits into each byte of data to

"synchronize" the communication.

� Uses less wires for communication (no clock signals)

� Simpler and more cost-effective.

� RS-232/RS-485/RS-422/TTL are the forms of Asynchronous

Communications.

� An asynchronous system is one in which each Character or Byte is

sent within a frame.

� Character-Oriented data transmission

� Transmitter and Receiver clocks are independent


Asynchronous System

� The receiver does not start detection until it receives the first bit,

known as the ‘Start Bit’.

� The start bit is in the opposite voltage state to the idle voltage and

allows the receiver to synchronize to the transmitter for the following

data in the frame.

� The receiver reads in the individual bits of the frame as they arrive,� The receiver reads in the individual bits of the frame as they arrive,

seeing either the logic 0 voltage or the logic 1 voltage at the

appropriate time.

� The ‘clock’ rate (Baud rate) at each end must be the same so that

the receiver looks for each bit at the time the transmitter sends it.

� Asynchronous communication can have problems at high speeds

(above 100 kbps).


Asynchronous Frame format


An asynchronous frame may have the following format:

� Start bit: Signals the start of the frame.

� Data: Usually 7 or 8 bits of data.

� Parity bit: Optional error detection bit.

� Stop bit(s): Usually 1, 1.5 or 2 bits.

A value of 1.5 means that the level is held for 1.5 times as long as for a single bit

Data CodingASCII (American Standard Code for Information Interchange)


Ex: D = ASCII code in binary 100 0100; Hex 44

Extended binary coded data interchange code (EBCDIC)


Example of serial date sending


Error detection method in asynchronous systems

� The simplest form of error checking in asynchronous systems is to

incorporate a parity bit, which may be even or odd.

� Even parity, the number of bits whose value is 1 in a given set are

counted. If that total is odd, the parity bit value is set to 1, making the

total count of 1's in the set an even number. If the count of ones in a

given set of bits is already even, the parity bit's value remains 0.

� In the case of odd parity, the situation is reversed. Instead, if the sum of

bits with a value of 1 is odd, the parity bit's value is set to zero. And if

the sum of bits with a value of 1 is even, the parity bit value is set to 1,

making the total count of 1's in the set an odd number.


Error detection method in asynchronous systems

EVEN - Parity bit set so that there is an even number of 1 bits

ODD - Parity bit set so that there is an odd number of 1 bits

NONE - Parity bit is ignored, value is indeterminate

MARK - Parity bit is ALWAYS set to 1

SPACE - Parity bit is ALWAYS set to 0


G I

EVEN - 0 0010 0111 1 0010 1001

ODD - 1 0010 0111 0 0010 1001

NONE - ? 0010 0111 ? 0010 1001

MARK - 1 0010 0111 1 0010 1001

SPACE - 0 0010 0111 0 0010 1001

LSB

Parity Parity

LSB

Synchronous System

� Synchronous Communication requires the sender and receiver to

share the same clock.

� The sender provides a timing signal to the receiver so that the

receiver knows when to "read" the data.

� Synchronous Communication generally has higher data rates and

greater error-checking capability.greater error-checking capability.

� A printer is a form of Synchronous Communication.

� The synchronous system packs many Characters or Bytes together

and sends them as a continuous stream, called a packet or a frame.

� Block-Oriented data transmission.

� Transmitter and Receiver clocks synchronized.


Synchronous System

� In synchronous systems, the receiver initially synchronizes to the

transmitter’s clock pulses, which are incorporated in the transmitted

data stream. This enables the receiver to maintain its

synchronization throughout large messages, which could typically

be up to 4500 bytes (36 000 bits). This allows large frames to be

transmitted efficiently at high data rates.

� The synchronous system packs many characters together and

sends them as a continuous stream, called a packet or a frame.

� DTE accepts a clock signal generated by DCE

� No start, stop bits, but still frame synchronization words are needed


Typical synchronous system frame format

Preamble: This comprises one or more bytes that allow the receiving unit

to synchronize with the frame.

SFD: The start of frame delimiter signals the beginning of the frame.


Destination: The address to which the frame is sent.

Source: The address from which the frame originated.

Length: The number of bytes in the data field.

Data: The actual message.

FCS: The frame check sequence is for error detection. Each of these is

called a field.

Synchronous System(I2C or I2C : Inter‐Integrated Circuit – Philips)

� 2-wire (TWI) – Serial data (SDA) and Serial clock (SCL)

� Common ground wire (totally 3 wires)

� Half-duplex, synchronous, multi-master Without “chip select” or

arbitration

‐


� Bus length: typical: inside the equipment, <1m, maximum: few meters

� There are no “slave‐select” lines – instead the devices have

“addresses” that are sent as part of the transmission protocol.

� Four max speeds (100 kbS (standard), 400 kbS (fast), 1 MbS (fast

plus), and 3.4 MbS (high‐speed)

I2C connection schematics


Synchronous SystemSPI (Serial Peripheral Interface ‐ Motorola)

• Two types of devices, masters and slaves.

– We’ll consider only one master, but multiple slaves.

• Signals (full duplex)


• Signals (full duplex)

– SCLK: Serial CLocK, set by Master

– MOSI: Master Out, Slave In

– MISO: Master In, Slave Out

– SS: Slave Select

• Each slave gets its own slave select (other lines are shared)

• Pulling line low selects slave

– Faster than I2C, (by few Mbps)

Serial data communication interface standards

� An interface standard defines the electrical and mechanical details

that allow equipment from different manufacturers to be connected

and able to communicate.

� The RS have produced several well known data interface standards:

�RS-232 and revisions�RS-232 and revisions

�RS-449 ( 37 pins )

�RS-423

�RS-422

�RS-485

�RS/TIA-530A ( Telecommunication Industry Association - TIA )

�RS/TIA-562


RS-423 and RS-422

� The RS-423 interface standard

� Is an Unbalanced system similar to RS-232 with increased

range and data transfer rates and up to 10 line receivers

per line driver.

� The RS-422 interface system

� Is a Balanced system with the same range as RS-423,

with increased data rates and up to 10 line receivers per

line driver.


Type of Serial data transmission lines

The choice between unbalanced and balanced

transmission lines is an important consideration when

selecting a data communications system.

� Unbalanced transmission line

� Balanced transmission line


Unbalanced transmission line

� In an unbalanced system, the signal common reference

conductor is simultaneously shared by many signals and

other electronic circuitry.

� Only one wire carries the signal voltage, which is

referenced to a signal common wire, sometimes called

the signal ground.

� The transmitted signal is the voltage between the signal

conductor and the common reference conductor.


Unbalanced transmission line cont�

� Theoretically, unbalanced transmission should work wellif the signal currents are small and the commonconductor has very low impedance.

� In practice, unbalanced systems only work over shortcommunication links.

� For long communication distances, the commonconductor does not have the same zero voltage at allpoints along its length or at its ends.points along its length or at its ends.

� The common conductor can also pick up noise and haveother voltages superimposed on it. Sometimes the shieldconductor is used as the common reference wire. Thispractice can introduce excessively high noise-levels andshould be avoided.

� Unbalanced transmission is used in the RS-232 and RS-423 interfaces.


Data communication with unbalanced interfaces


Balanced transmission line

� Balanced communication interfaces require two

conductors to transmit each signal.

� The voltage at the receiving end is measured as the

voltage difference between these two wires. This is

known as a balanced or differential system.

� This eliminates many of the interference problems

associated with the common reference wire.


Data communications with balanced interfaces


Balanced transmission line cont�

� The balanced transmission line permits a higher rate of

data transfer over longer distances.

� The differential method of data transfer is preferable in

industrial applications where noise can be a major

problem.problem.

� The disadvantage is that a balanced system requires two

conductors for every signal.

� Balanced transmission is used in most of the fast

interfaces such as RS-422 and RS-485.


RS-485 interface standard

� The RS-485 is a balanced system with the same range

as RS-422, but with increased data rates and up to 32

transmitters and receivers possible per line.

� RS-485 allows higher speeds and much greater

distances than RS-232. Depending on their design, up to

32 devices can be connected together on a single pair of

wires, in Multidrop and/or Multimaster configuration.


Multidrop

� In a Multidrop communication system, more then two

devices are connected together on a single transmission

medium. In such a system, each device must have a

unique address.


Multimaster

� Multimaster communication system can have more then

one master device in network. For this to work, the

masters need a specified way to allow each other an

opportunity to transmit.

– The RS-485 interface standard is very useful for

instrumentation and control systems.


Multidrop Network

Half-Duplex Network Using Terminating Resistors


Full-Duplex Network Using Terminating Resistors

Pin Functions for RS-232/RS-485/RS-422

D-shell 9-pin connector


NPort IA5150 4W/2W RS-485/RS-422 Pin Assignments


RS-232/ RS-422/ RS-485


Serial interface converters

EIA - Electronic Industries Association


Block structure of RS-232/RS-485 converter

Documents

01 RS232 Commm