Digital Interfaces

• An interface links two devices• Interface Standards define:

– mechanical specifications - how many wires & connector type

– electrical specifications - frequency, amplitude and phase of signal

– functional - what does each wire do?– Procedural – how & when to perform functions

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Serial vs. Parallel• In serial transmission one bit is sent with

each clock pulse.

• Two types of serial transmission:– asynchronous– synchronous

• In parallel transmission multiple bits are sent with each clock pulse. Faster/more expensive

Serial Transmission

0 1 1 0 0 0 1 0

Parallel Transmission01100010

01100010

Serial Transmission

• Asynchronous & Synchronous

• Concerned with timing issues

• How does the receiver know when the bit period begins and ends?

• Small timing difference become more significant over time if no synchronization takes place between sender and receiver

Synchronizations

1 0 0 1 0 0 1 1 1 1 1 0

1 0 0 1 1 0 0 1 1 1 1 1 1 0

time

time

Sen

der

cloc

k R

ecei

ver

cloc

k

Sender data

Received data

Asynchronous Transmission

• Serial communication• Data transmitted 1

character at a time• Character format is 1

start & 1 or more stop bits, plus data of 5-8 bits

• Character may include parity bit

• Timing needed only within each character

• Resynchronization each start bit

• Uses simple, cheap technology

• Wastes 20-30% of bandwidth

Asynchronous Transmission

STOP BIT

START BIT

0 1 1 0 0 0 1 00 1 1 0 0 0 1 0 0 1 1 0 0 0 1 0

Gaps of indeterminate size

Synchronous Transmission

• Serial communication• Large blocks of bits

transmitted without start/stop codes

• Synchronized by clock signal or clocking data

• Data framed by preamble/post amble bit patterns

• More efficient than asynchronous

• Overhead typically below 5%

• Used at higher speeds than asynchronous

Synchronous Transmission

0 1 1 0 0 0 1 00 1 1 0 0 0 1 0 0 1 1 0 0 0 1 0 0 1 1 0 0 0 1 0 0 0 1 0

Synchronization Choices

• Low-speed terminals and PCs commonly use asynchronous transmission– inexpensive– “burst” tendency of communication reduces impact of

inefficiency

• Large systems and networks commonly use synchronous transmission– overhead too expensive; efficiency necessary– error-checking more important

Generic Communications Interface Illustration

DTE DCE DTE

Generates Data

Converts totransmission

media/ converts to generated data

ReceivesData

(Network)DCE

Converts totransmission

media/ converts to generated data

RS-232C (EIA 232C)

• EIA’s “Recommended Standard” (RS)

• Specifies mechanical, electrical, functional, and procedural aspects of the interface

• Used for connections between DTEs and voice-grade modems, and many other applications

Mechanical Specifications

• 25-pin connector with a specific arrangement of leads

• DTE devices usually have male DB25 connectors while DCE devices have female

• In practice, fewer than 25 wires are generally used in applications

DB-25 Female

DB-25 Male

RS-232 DB-25 Connectors

RS-232 DB-9 Connectors

Limited RS-232

Electrical Specifications

• Specifies signaling between DTE and DCE

• Uses NRZ-L encoding– Voltage -15V to -3V = binary 1– Voltage +15V to +3V = binary 0

• Rated for < 20Kbps and < 15Mts– greater distances and rates are theoretically

possible, but not necessarily wise

Functional Specification

• Specifies the role of the individual circuits• Data circuits in both directions allow full-

duplex communication• Timing signals allow for synchronous

transmission (although asynchronous transmission is more common)

Functional Specification - DB-25 Pinouts

Procedural Specification

• Multiple procedures are specified

• Provides means of attachment between computer and modem– Specifies method of transmitting data between

devices– Specifies method of cooperation for exchange of

data between devices– Handshaking between the two equipment

Null Modem Cable

SGDTR

DSR

RTS

CTSCDTDRD

SGDTR

DSR

RTS

CTSCDTDRD

• Allows DTE to DTE direct communication

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Modems

• Modulator - converts digital signal to analog signal

• Demodulator - converts analog signal to digital signal

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Data Rate

• Encoding Technique

• Physical properties of medium

• Can increase speed by increasing frequency but every line has upper and lower limits

• frequency range = bandwidth

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Bit Rate Summary

Encoding HDX FDXASK, FSK, 2-PSK 2400 12004-PSK, 4-QAM 4800 24008-PSK, 8-QAM 7200 360016-QAM 9600 480064-QAM 14,400 7200256-QAM 19,200 9600