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Interface DesignInterface Design

Serial CommunicationsSerial Communications

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OutlineOutline

Concept of serialConcept of serial

communicationscommunications

Synchronous, AsynchronousSynchronous, Asynchronous

RSRS--232232 standardstandard

Hand shakingHand shaking

UART and USART chi sUART and USART chi s

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What is Serial CommunicationsWhat is Serial Communications

LONG

Non-Distort

Non-Expensiveshort

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Serial CommunicationSerial Communication

TypesTypesAsynchronous (transfer singleAsynchronous (transfer single

byte at a time)byte at a time)

Synchronous (block of data, chr.,Synchronous (block of data, chr.,

at a time)at a time)

Transfer:Transfer:

SimplexSimplex

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chipsetchipset

UART, e.g.UART, e.g. 82508250 COM portCOM port

USART, IntelUSART, Intel82518251

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Transfer TypesTransfer Types

Simplex, Half- and Full-Duplex Transfers

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Asynchronous Data FramingAsynchronous Data Framing

Idle (high): Mark

Low: Space

Overhead? (parity, start, stop)

Notransfer

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Data Transfer RateData Transfer Rate

Baud rateBaud rateFor Modems, # signal changes/secondFor Modems, # signal changes/second

In lectures, same as bpsIn lectures, same as bps

bpsbps

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VocabularyVocabulary

DTEDTEData Terminal EquipmentData Terminal Equipment

e.g. computer, terminale.g. computer, terminal

DCEDCEData Communication EquipmentData Communication Equipment

connects DTE to communication linesconnects DTE to communication lines

e.g. modeme.g. modem

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RSRS--232232 StandardStandard

Electronics Industries AssociationElectronics Industries AssociationEIAEIA 232232

Specifies the interface betweenSpecifies the interface betweenDTE and DCEDTE and DCE

Even used in applications whereEven used in applications where

there is no DCEthere is no DCE e.g. connecting computer to printer, magnetice.g. connecting computer to printer, magnetic

card reader, robot,card reader, robot, etc.etc.

Introduced inIntroduced in 19621962 but is stillbut is still

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MechanicalMechanical

CharacteristicsCharacteristics2525--pin connectorpin connector

99--pin connector is more commonly found inpin connector is more commonly found inIBMIBM--PC but it covers signals for asynchronousPC but it covers signals for asynchronous

serial communication onlyserial communication only

Use male connector on DTE andUse male connector on DTE andfemale connector on DCEfemale connector on DCE

Note: all signal names areNote: all signal names areviewed from DTEviewed from DTE

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2525--Pin RSPin RS232232 ConnectorConnector

Source: Duck, Bishop & Read, Data Communications for Engineers, Addison-Wesley

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99--Pin RSPin RS232232 ConnectorConnector

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DTE ConnectionsDTE Connections

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Electrical CharacteristicsElectrical Characteristics

SingleSingle--endedended one wire per signal, voltage levels are withone wire per signal, voltage levels are withrespect to system common (i.e. signal ground)respect to system common (i.e. signal ground)

Mark:Mark: 33V toV to 2525VV represent Logicrepresent Logic 11, Idle State (OFF), Idle State (OFF)

Space: +Space: +33 to +to +2525VV represent Logicrepresent Logic 00, Active State (ON), Active State (ON)

Usually swing betweenUsually swing between 1212V toV to++1212VV

Recommended maximum cableRecommended maximum cablelen th islen th is 1515m atm at 2020kb skb s

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Function of SignalsFunction of Signals

TD: transmitted dataTD: transmitted data

RD: received dataRD: received data

D

SR: data set readyD

SR: data set ready indicate whether DCE is powered onindicate whether DCE is powered on

DTR: data terminal readyDTR: data terminal ready indicate whether DTR is powered onindicate whether DTR is powered on

turning offDTR causes modem to hang up theturning offDTR causes modem to hang up thelineline

RI: ring indicatorRI: ring indicator

ON when modem detects phone callON when modem detects phone call

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Function of SignalsFunction of Signals

DCD: data carrier detectDCD: data carrier detect ON when two modems negotiated successfullyON when two modems negotiated successfully

and carrier signal is established on the phoneand carrier signal is established on the phone

lineline

RTS: request to sendRTS: request to send ON when DTE wants to send dataON when DTE wants to send data

Used to turn on and off modemUsed to turn on and off modems carrier signals carrier signalin multiin multi--point (i.e. multipoint (i.e. multi--drop) linesdrop) lines

Normally constantly ON in pointNormally constantly ON in point--toto--point linespoint lines

CTS: clear to sendCTS: clear to send ON when DCE is ready to receive dataON when DCE is ready to receive data

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Flow ControlFlow Control

Means to ask the transmitter toMeans to ask the transmitter tostop/resume sending in datastop/resume sending in data

Required when:Required when: DTE to DCE speed > DCE to DCE speedDTE to DCE speed > DCE to DCE speed

(e.g. terminal speed =(e.g. terminal speed = 115115..22kbps and linekbps and linespeed =speed = 3333..66kbps, in order to benefit fromkbps, in order to benefit from

modemmodems data compression protocol)s data compression protocol) without flow control, the buffer within modem willwithout flow control, the buffer within modem will

overflowoverflow sooner or latersooner or later

the receiving end takes time to process thethe receiving end takes time to process thedata and thus cannot be always ready todata and thus cannot be always ready toreceivereceive

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Hardware Flow ControlHardware Flow Control

RTS/CTSRTS/CTS the transmitting end activates RTS to informthe transmitting end activates RTS to inform

the receiving end that it has data to sendthe receiving end that it has data to send

if the receiving end is ready to receive, itif the receiving end is ready to receive, itactivates CTSactivates CTS

normally used between computer and modemnormally used between computer and modem

computer is always ready to receive data but modemcomputer is always ready to receive data but modem

is not, because terminal speed > link speedis not, because terminal speed > link speed

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Other StandardsOther Standards

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ProgrammableProgrammable

Communications Interface:Communications Interface:

UARTUART 82508250//1655016550

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TheThe 1655016550 can control a modemcan control a modemthrough DSR, DTR, CTS, RTS, RIthrough DSR, DTR, CTS, RTS, RI

and DCD.and DCD. In this context, the modem isIn this context, the modem is

called the data set while thecalled the data set while the

1655016550 is called the datais called the dataterminal.terminal.

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Pinout of thePinout of the 1655016550AA00, A, A11 and Aand A22: Select an: Select an

internal register forinternal register forprogramming and dataprogramming and datatransfer.transfer.

ADSADS:: Address strobe,Address strobe, used toused tolatch address and chip select.latch address and chip select.

BAUDOUTBAUDOUT: Clock signal from: Clock signal fromBaud rate generator inBaud rate generator in

transmitter.transmitter.

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Pinout of thePinout of the 1655016550 DD77--DD00: Data bus pins: Data bus pins

connected to processor dataconnected to processor databusbus

DCDDCD:: Data carrier detectData carrier detect used by modem to signalused by modem to signal 1655016550 thatthat

carrier is presentcarrier is present

DDISDDIS:: Disable driver outputDisable driver output

Set toSet to 00 to indicate that processor isto indicate that processor isreading data from the UARTreading data from the UART Used to change direction of data flowUsed to change direction of data flow

through a bufferthrough a buffer

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Pinout of thePinout of the 1655016550 contdcontd

DSRDSR:: Data set readyData set ready is inputis inputtoto 1655016550 indicates that modem (data set) is readyindicates that modem (data set) is ready

to operateto operate DTRDTR:: Data terminal readyData terminal ready isis

outputoutput indicates that data terminal (indicates that data terminal (1655016550) is) is

ready to functionready to function INTRINTR:: Interrupt requestInterrupt request isis

output to processor tooutput to processor torequest an interruptrequest an interrupt

Data receivedData received

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Pinout of thePinout of the 1655016550 MRMR:: Master resetMaster reset, connect to, connect to

system RESETsystem RESET OUTOUT11, OUT, OUT22: User defined: User defined

output pins for modem oroutput pins for modem orother device.other device. RCLKRCLK: Receiver clock,: Receiver clock,

clock input to receiver section of theclock input to receiver section of the

UARTUART alwaysalways 1616X the desired receiver BaudX the desired receiver Baud

raterate

RDRD: Read inputs: Read inputs cause data to be read from register givencause data to be read from register given

by the address inputsby the address inputs

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Pinout of thePinout of the 1655016550 RTSRTS: Request: Request--toto--send signalsend signal

to modemto modem indicating UART wishes to send dataindicating UART wishes to send data

SIN, SOUTSIN, SOUT: Serial data pins,: Serial data pins,in and outin and out RxRDYRxRDY: Receiver ready: Receiver ready

transfer received data via DMAtransfer received data via DMA

techniquestechniques TxRDYTxRDY: Transmitter ready: Transmitter ready

transfer transmitter data via DMAtransfer transmitter data via DMA

WRWR: Write: Write connects to micro write signal to transferconnects to micro write signal to transfer

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82508250//1645016450//1655016550 UARTUART

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UART in PCUART in PC

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RegistersRegisters

Transmitter holding registerTransmitter holding register

Receiver buffer registerReceiver buffer register

Interrupt enable registerInterrupt enable register

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RegistersRegisters

Interrupt identification registerInterrupt identification register

C (C (

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Line Control (DataLine Control (Data

Format) RegisterFormat) Register

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Modem RegistersModem Registers

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Line Status RegisterLine Status Register

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Divisor RegisterDivisor Register

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Synchronous ProtocolsSynchronous Protocols

(Serial Data Link Control)(Serial Data Link Control)

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CRCCRC

In SDLC:

G(X) = x**16 + x**12 + x**5 + 1

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82518251 Block DiagramBlock Diagram