Digital Interfaces
•An interface links two devices
•Interface Standards define:
– mechanical specifications - how many wires &connector type
–electrical specifications - frequency, amplitudeand 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 witheach clock pulse.
•Two types of serial transmission:
–asynchronous
–synchronous
•In parallel transmission multiple bits aresent with each clock pulse. Faster/moreexpensive
Serial Transmission
0 1 1 0 0 0 1 0
Parallel Transmission
01100010
01100010
Serial Transmission
•Asynchronous & Synchronous
•Concerned with timing issues
•How does the receiver know when the bitperiod begins and ends?
•Small timing difference become moresignificant over time if no synchronizationtakes place between sender and receiver
Synchronizations
Asynchronous Transmission
•Serial communication
•Data transmitted 1character at a time
•Character format is 1start & 1 or more stopbits, plus data of 5-8bits
•Character may includeparity bit
•Timing needed onlywithin each character
•Resynchronizationeach start bit
•Uses simple, cheaptechnology
•Wastes 20-30% ofbandwidth
Asynchronous Transmission
STOPBIT
STARTBIT
0 1 1 0 0 0 1 0
0 1 1 0 0 0 1 0
0 1 1 0 0 0 1 0
Gaps ofindeterminate size
Synchronous Transmission
•Serial communication
•Large blocks of bitstransmitted withoutstart/stop codes
•Synchronized by clocksignal or clocking data
•Data framed bypreamble/post amblebit patterns
•More efficient thanasynchronous
•Overhead typicallybelow 5%
•Used at higher speedsthan asynchronous
Synchronous Transmission
0 1 1 0 0 0 1 0
0 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 useasynchronous transmission
–inexpensive
–“burst” tendency of communication reducesimpact of inefficiency
•Large systems and networks commonly usesynchronous transmission
–overhead too expensive; efficiency necessary
–error-checking more important
Generic CommunicationsInterface Illustration
DTE
DCE
DTE
GeneratesData
Converts totransmissionmedia/ convertsto generated data
ReceivesData
(Network)
DCE
Converts totransmissionmedia/ convertsto 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 andvoice-grade modems, and many otherapplications
MechanicalSpecifications
•25-pin connector with a specificarrangement of leads
•DTE devices usually have male DB25connectors while DCE devices have female
•In practice, fewer than 25 wires aregenerally used in applications
DB-25 Female
DB-25 Male
RS-232 DB-25Connectors
RS-232 DB-9Connectors
Limited RS-232
ElectricalSpecifications
•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 theoreticallypossible, 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 synchronoustransmission (although asynchronoustransmission is more common)
Functional Specification -DB-25 Pinouts
Procedural Specification
•Multiple procedures are specified
•Provides means of attachment betweencomputer and modem
–Specifies method of transmitting data betweendevices
–Specifies method of cooperation for exchange ofdata between devices
–Handshaking between the two equipment
Null Modem Cable
SG
DTR
DSR
RTS
CTS
CD
TD
RD
SG
DTR
DSR
RTS
CTS
CD
TD
RD
•Allows DTE to DTE direct communication
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Modems
•Modulator - converts digital signal toanalog signal
•Demodulator - converts analog signal todigital signal
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Data Rate
•Encoding Technique
•Physical properties of medium
•Can increase speed by increasing frequencybut every line has upper and lower limits
•frequency range = bandwidth
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Bit Rate Summary
