Interfaces%20and%20Synchronization

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Interfaces and Synchronization

• Martin Weiss

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EIA 232D Interface Standard

• Synonymous with ITU V.24
• Asynchronous interface
• Up to 19.2kbps
• 50 foot maximum distance between DCE and DTE

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Other Interface Standards

• EIA 449 - Uses EIA-422 (balanced) or EIA-423
  (unbalanced) signalling standards
• EIA 530 - Same as EIA 449 except with a 25 pin
  connector

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Signalling Standards
Circuit Type
Max. Separation (m)
Max. Bit Rate
EIA-423
10 100 1000
100kbps 10kbps 1kbps
EIA-422
10 100 1000
10Mbps 1Mbps 100kbps
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Main EIA 232D Signals

• Receive data (RxD) - Pin 2
• Transmit data (TxD) - Pin 3
• Request to Send (RTS) - Pin 4
• Clear to Send (CTS) - Pin 5

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Main EIA 232D Signals

• Data Set Ready (DSR) - Pin 6
• Data Terminal Ready (DTR) - Pin 20
• Ring Indicator (RI) - Pin 22
• Carrier Detect (CD) - Pin 8

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EIA 232D Protocol
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Questions

• For data transfer, when would we want to transfer
  bits one at a time, and when as a group?
• Why do we have to worry about synchronization?
• How can we begin to quantify performance issues?

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Bit Synchronization

• The receiver must know when a bit starts and when
  it stops
• Normally, synchronization sequences are necessary

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Character Synchronization

• When does a character start?

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Asynchronous Communications

• Characters are transmitted when they are
  generated
• The receiver must be configured so that the gross
  transmission characteristics are identical to the
  transmitter

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Implementation of Async

• Line is normally in a logical High state
• Preceding a character is a Start Bit (Low for
  one bit time)
• Subsequent bits may be low or high
• Character may be ended by stop bit(s)

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Implementation of Async
Start
0
0
0
0
1
1
1
Stop
Stop
Parity ?
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Discussion of Async

• Very simple to implement
• Inefficient
• Alternative synchronous transmission

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Discussion of Async
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Synchronous Transmission

• Transmitter and receiver are synchronized at the
  bit and character level prior to transmission
• Messages may still arrive asynchronously
• Synchronous systems are normally more complex

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Performance Analysis of Communications Links

• Consider the effect of propagation delay and
  transmission rate
• Recall that

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Definitions

• Let U (throughput)/(capacity) T/R
• Throughput is the number of bits actually
  transmitted per unit time
• Capacity is the number of bits that could be
  transmitted per unit time
• R data rate of the channel
• d maximum distance between any two stations

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Definitions

• V velocity of signal propagation
• L frame length (average or fixed)
• T throughput
• Assuming no overhead,

Number of bits
T
Time from source to destination
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Illustration (a lt 1)
t0
Start of Transmission
t0a
Start of Reception
t01
End of Transmission
t01a
End of Reception
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Illustration (a gt 1)
t0
Start of Transmission
t01
End of Transmission
t0a
Start of Reception
t01a
End of Reception
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Utilization of a Baseband Bus
Utilization Throughput/Capacity
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U vs. a
U
1.0
0.8
0.6
0.4
0.2
a
1
5
10
15
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