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OSI Physical Layer

Components

Digital and Data Communications

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Data Communication Link

DTE DCE DCE DTE

RS -232C RS -232C

Transmission Medium

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UART Uses

PC serial port is a UART! Serializes data to be sent over serial cable

De-serializes received data

Serial

Cable

Serial

Cable

Device

SerialPort SerialPort

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UART Uses

Communication between distant computers Serializes data to be sent to modem

De-serializes data received from modem

Serial

Cable

Serial

CablePhone

Line

Phone

Line

Modem

Modem

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Options Found in UARTs

Character Type Option for an 8-bit bus

No. of Stop bits required

Use of parity type

Use of diff. data rates

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ACIA

AsynchronousCommunications

Interface Adapter

by Motorola

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UNITS of COMMUNICATION LINK

UART

Transmitter side

needs 8 data bus lines for a 8-bit parallel

data to be converted into a serial data

stream.

Idle line one/marking state

Some are onboard

Example ACIA by Motorola

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UART Transmission Example

Send the ASCII letter W (1010111)

1

Line idling Start bit Parity bit(odd parity) Stop bit

Line idling again

Mark

Space

7 data bits Least significant bit first

1 1 0 1 0 1 0

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MC6850Pin Assignment

CPU Pin

Data Bus (D0-D7)

Control Bus (R/W, E, CS)

Address (RS, CS)

Other: IRQ

EIA-232 Pin

Data (Rx, Tx)

Control (RTX, CTS, DCD)

Baud Clock (Rx CLK, Tx CLK)

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ACIA Register Contents

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ACIA Control Register (8-bit)

7 6 5 4 3 2 1 0

RX

IR

__

RTS

TX

IR

WORD

SIZE

STOP

&

PARITY

COUNTER

DIVIDE

AC A C l i ( bi )

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ACIA Control Register (8-bit)

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UART Receiver

Use of Divide ratio

In half of the bit period where a start

bit can be verified

Use of shift register

Clock Skew

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UARTStatus Register

7 6 5 4 3 2 1 0

__

IRQ

PE OVRN FE __

CTS

__

DCD

TDRE RDRF

Ch ki UART P f

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Checking UART Performance

Use of Status Register Use of TDRE

From buffer register to the shift-register

Use of RDRF

From shift-register to the buffer register

Use of OVRN

failure to read the data from the buffer register

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Checking UART Performance

Use of Parity Error Use of Framing Error

Use of IRQ

2 conditions when IR functions at the

Control register

TDRE or RDRF is high

a received error has occurred

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RS-232C Specifications

Most common by EIA

Uses a D-type 25 pin connector

uses voltage levels of +3 to +25 V (SPACE,data 0, on, green) and -3 to -25 V (MARK, data

1, off, red)

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RS-232C Specifications

Cable impedance

330 min Tx

7000 max - Rx

Line Capacitance -2500 pf

50 pf/ foot limited to 100kb/s

Ideal for short distance less than 50ft

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RS-232C

Since the link only uses about 7 wires, DB-9

connectors are frequently used now for seriallinks,

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Handshaking Lines

DTE uses a signal called RTS on pin 4, while

DCE uses a signal called CTS on pin 5.

Data is transmitted from DTE to DCE on pin 2,

and from DCE to DTE on pin 3.

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NULL Modem

1

2

3

4

5

6

7

8

20

1

2

3

45

6

7

8

20

DTE 1 DTE 2

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RS-449

Uses D-type

connectors

37-pin

9-pin

2 Mbps transfer rate Max length -200 feet

w/ loop-back feature

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RS-422 balanced electrical interface

implemented using a D-type connector with 37 pins RS-449 specifications

logic 1 (Mark) is represented by the A output beingmore negative than the B output.

logic 0 (Space) is represented by the A output being

more positive than the B output.

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RS-423

unbalanced electrical interface

implemented using a D-type connector with 37pins RS-449 specifications

low speed communications

generally operate at +/- 6 VDC and are

compatible with RS-232 devices

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RS-423 very sensitive, capable of detecting Mark/Space

states at +/- 0.4 VDC

has separate Signal Grounds for transmitters andreceivers

A binary one (Mark) is represented by a Negativevoltage level

and a binary zero (Space) is represented by a

Positive voltage level.

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ASSIGN:

USB interface standards How to interface a PC to PC using USB port?