Digital%20Interfaces

1
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

2
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

3
Serial Transmission
0 1 1 0 0 0 1 0
4
Parallel Transmission
01100010
01100010
5
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

6
Synchronizations
7
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

8
Asynchronous Transmission
STOP BIT
START BIT
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 of indeterminate size
9
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

10
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
11
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

12
Generic Communications Interface Illustration
DTE
DTE
DCE
DCE
(Network)
Converts totransmissionmedia/ converts to
generated data
Generates Data
Converts totransmissionmedia/ converts to
generated data
ReceivesData
13
RS-232C (EIA 232C)

• EIAs 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

14
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

15
RS-232 DB-25 Connectors
16
RS-232 DB-9 Connectors

• Limited RS-232

17
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 lt 20Kbps and lt 15Mts
• greater distances and rates are theoretically
  possible, but not necessarily wise

18
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)

19
Functional Specification - DB-25 Pinouts
20
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

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Null Modem Cable

• Allows DTE to DTE direct communication

22
Modems

• Modulator - converts digital signal to analog
  signal
• Demodulator - converts analog signal to digital
  signal

23
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