Digital Interfaces An interface links two devices Interface

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 upated 1/2002 1

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 upated 1/2002 2

Serial Transmission 0110 0010 upated 1/2002 3

Parallel Transmission 0 1 1 0 0 0 1 0 upated 1/2002 4

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 upated 1/2002 5

Synchronizations upated 1/2002 6

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 upated 1/2002 7

Asynchronous Transmission START BIT 0110 0010 STOP BIT 0110 0010 Gaps of indeterminate size upated 1/2002 8

Synchronous Transmission • Serial communication • Data framed by preamble/post amble • Large blocks of bits bit patterns transmitted without • More efficient than start/stop codes asynchronous • Synchronized by clock • Overhead typically signal or clocking data below 5% • Used at higher speeds than asynchronous upated 1/2002 9

Synchronous Transmission 0110 0010 0010 upated 1/2002 10

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 upated 1/2002 11

Generic Communications Interface Illustration DTE Generates Data DCE (Network) DCE Converts to transmission media/ converts to generated data upated 1/2002 DTE Receives Data 12

RS-232 C (EIA 232 C) • 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 upated 1/2002 13

Mechanical Specifications • 25 -pin connector with a specific arrangement of leads • DTE devices usually have male DB 25 connectors while DCE devices have female • In practice, fewer than 25 wires are generally used in applications upated 1/2002 14

RS-232 DB-25 Connectors DB-25 Female DB-25 Male upated 1/2002 15

RS-232 DB-9 Connectors Limited RS-232 upated 1/2002 16

Electrical Specifications • Specifies signaling between DTE and DCE • Uses NRZ-L encoding – Voltage -15 V to -3 V = binary 1 – Voltage +15 V to +3 V = binary 0 • Rated for < 20 Kbps and < 15 Mts – greater distances and rates are theoretically possible, but not necessarily wise upated 1/2002 17

Functional Specification • Specifies the role of the individual circuits • Data circuits in both directions allow fullduplex communication • Timing signals allow for synchronous transmission (although asynchronous transmission is more common) upated 1/2002 18

Functional Specification DB-25 Pinouts upated 1/2002 19

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 upated 1/2002 20

Null Modem Cable • Allows DTE to DTE direct communication SG DTR DSR RTS CD TD RD upated 1/2002 SG DTR DSR RTS CD TD RD 21

Modems • Modulator - converts digital signal to analog signal • Demodulator - converts analog signal to digital signal upated 1/2002 22

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 upated 1/2002 23

Bit Rate Summary upated 1/2002 24