Chapter 5' Local Asynchronous Communication RS232

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Chapter 5. Local Asynchronous Communication
(RS-232)

• Jing Wang
• Towson University

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5.1. Introduction

• How to use electric current to transfer digital
  information across short distances
• How bits can be encoded
• Binary digits (bits)
• mechanism to send characters between keyboard and
  computer
• Quantitative measures
• Bandwidth and delay

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5.2. The Need For Asynchronous Communication
(RS-232)

• Asynchronous
• The sender and receiver do not synchronize before
  each transmission
• A sender transmits at any time
• A receiver accepts and interprets the signal
• Useful for devices such as keyboards

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5.3. Using Electric Current To Send Bits

• Figure 5.1. Illustration of how positive and
  negative voltage can be used to transmit bits
  across a wire. In this example, the sender
  applies a negative voltage to send a 1 bit or a
  positive voltage to send a 0 bit.

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5.3. Using Electric Current To Send Bits

• Waveform diagram
• A visual representation of how an electrical
  signal varies over time

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5.4. Standards For Communication

• Organizations
• ITU (International Telecommunication Union)
• EIA (Electronic Industries Association (EIA)
• IEEE (Institute for Electrical and Electronic
  Engineers)

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5.4. Standards For Communication (cont.)

• RS-232
• A standard produced by EIA
• Transfer characters across copper wires between a
  computer and a device such as a modem, keyboard,
  or terminal
• Specifies details of physical connection
• Connection less than 50 feet long
• The two voltages range from -15 volts to 15 volts

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5.4. Standards For Communication (cont.)

• RS-232
• Defines serial, asynchronous communication
• Serial (bits travel one after another)
• Never leaves zero volts on the wire
• When the transmitter has nothing to send, it
  leaves the wire with a negative voltage
  (corresponds to bit value 1)
• Start bit - 0
• Stop bit 1
• Transmission of 7 bits character requires 9 bits

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5.4. Standards For Communication (cont.)

• Figure 5.2. The voltage on a wire as a character
  is transmitted using RS-232. A start bit notifies
  the receiver that a character is starting, and
  each bit transmission lasts the same length of
  time.

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5.4. Standards For Communication (cont.)

• RS-232 is a popular standard used for
  asynchronous, serial communication over short
  distances between a computer and a modem or ASCII
  terminal.
• RS-232 precedes each character with a start bit,
  follows each character with an idle period at
  least one bit long (stop bit), and sends each bit
  in exactly the same length of time.

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5.4. Standards For Communication (cont.)

• Types of local communication
• Determining factors Cost and Ease of Use
• IEEE 1394 high speed serial bus (Fire Wire)
• Speed up to 400Mbps
• Self-configured addressing
• A tiered-star topology allow up to 63 devices

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5.4. Standards For Communication (cont.)

• USB (Universal Serial Bus)
• Easy to use for end users
• Wide range of workload and applications
• Bandwidths ranging from a few kbps to hundreds
  Mbps
• Low-speed 10 100 Kbps (keyboard, mouse)
• Full-speed 500 Kbps 10 Mbps (phone, audio)
• High-speed 25 400 Mbps (video, storage)
• Support up to 127 physical devices
• Flexibility different packet size, data rate,
  flow control
• Robustness hot swap, error handling
• SCSI (Small Computer Systems Interface)

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5.4. Standards For Communication (cont.)

• RS-232
• RS-422/485
• Balanced systems
• Balanced transmission each of the main circuits
  requires two wires, with no common ground
• Unbalanced transmission all circuits share a
  common ground
• Speed over 10 Mbps
• Connect up to 32 devices
• Distance up to 1200m

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Figure 6-15
RS-423 Unbalanced Mode
NRZ-L nonreturn to zero-level
? The McGraw-Hill Companies, Inc., 1998
WCB/McGraw-Hill
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Figure 6-16
RS-422 Balanced Mode
NRZ-L nonreturn to zero-level
? The McGraw-Hill Companies, Inc., 1998
WCB/McGraw-Hill
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5.5. Baud Rate, Framing, And Errors

• Baud rate
• The number of changes in the signal per second
  that the hardware generates
• RS-232 baud rate equal to bit rate
• Framing errors
• A receiver measures the voltage for each bit
  multiple times and compares the measurements.
• If the voltages do not all agree or if the stop
  bit does not occur exactly at the time expected,
  the receiver reports an error.

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5.6. Full Duplex Asynchronous Communication

• Figure 5.3. The minimal wiring required for
  full-duplex RS-232 communication in which control
  wires are omitted. Although the two circuits
  carry data independently, it is possible for them
  to share a single ground wire.

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5.6. Full Duplex Asynchronous Communication

• Electric circuits requires two wires
• Signal (current flows out)
• Ground (return path)
• Transmission type
• Full duplex
• Half duplex or simplex

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5.7. Limitations of Real Hardware

• Figure 5.4. An illustration of the voltage
  emitted by a real device as it transmits a bit.
  In practice, voltages are often worse than this
  example.

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5.7. Limitations of Real Hardware

• Impairment
• Attenuation
• loss of energy
• Distortion
• Signal changes its form or shape
• Occurs in a composite signal
• Noise
• Thermal noise random motion of electrons in a
  wire
• Induced noise from motors and appliances
• Crosstalk the effect of one wire on the other
• Impulse noise a signal with high energy in a
  very short time

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5.8. Hardware Bandwidth And The Transmission of
Bits

• Bandwidth
• The maximum rate that hardware can change a
  signal
• Measured in cycles per second or Hertz (Hz)
• Nyquist Intersymbol Interference Theorem
• If the transmission system of bandwidth B uses K
  possible values of voltage, the maximum data rate
  in bits per second D is
• D 2Blog2K
• RS-232, K 2, so D 2B

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5.9. The Effect of Noise On Communication

• Shannons Theorem
• C Blog2(1 S/N)
• C is effective limit on the channel capacity in
  bits per second
• B is the hardware bandwidth
• S is the average signal power
• N is the average noise power
• Noise is background interference
• S/N is signal-to-noise ratio
• 10log10S/N is decibels (dB)

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5.10. Significance For Data Networking

• Nyquists theorem encourages engineers to explore
  ways to encode bits on a signal because a clever
  encoding allows more bits to be transmitted per
  unit time.
• Shannons Theorem informs engineers that no
  amount of clever encoding can overcome that laws
  of physics that place a fundamental limit on the
  number of bits per second that can be transmitted
  in a real communication system

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5.10. Significance For Data Networking

• Voice telephone system has a signal-to-noise
  ratio of approximately 30 dB and a bandwidth of
  approximately 3000 Hz
• 10log10S/N 30 dB
• S/N 1000
• According to Shannons Theorem, the maximum
  number of bits per second that can be transmitted
  is limited to
• C 3000log2(1 1000)
• Approximately 30,000 bps

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5.11. Summary

• RS-232 standard
• Asynchronous transmission of characters over
  short distances
• Between a keyboard and a computer
• Over a computers serial ports
• Idle negative voltage
• Start bit informs a character is arriving
• Stop bit detects all bits of the character
  arrived in the time allotted.

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5.11. Summary

• Bandwidth
• Hardware bandwidth the speed at which hardware
  can change state
• Nyquists theorem defines relationship between
  hardware bandwidth and the theoretical maximum
  rate at which data can be sent
• Shannons theorem gives a limit on the rate at
  which data can be sent in the presence of noise

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