Fall 2005 Local Serial Asynchronous Communication

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Fall 2005Local Serial Asynchronous Communication

• Qutaibah Malluhi
• Computer Science and Engineering
• Qatar University

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Bit-wise data transmission

• Data transmission requires
• Encoding bits as energy
• Transmitting energy through medium
• Decoding energy back into bits
• Energy can be electric current, radio, infrared,
  light
• Transmitter and receiver must agree on encoding
  scheme and transmission timing

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Using Electric Current to Send Data

• Simple idea - use varying voltages to represent
  1s and 0s
• One common encoding use negative voltage for 1
  and positive voltage for 0
• In following figure, transmitter puts positive
  voltage on line for 0 and negative voltage on
  line for 1

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Encoding Details and Standards

• Encoding scheme must specify the details. For
  example
• How long will voltage last for each bit?
• How soon will next bit start?
• How will the transmitter and receiver agree on
  timing?
• All details specified by standards
• Allow interoperability of devices adhering to the
  standard
• Several organizations produce networking
  standards
• International Telecommunications Union (ITU)
• Electronic Industries Association (EIA)
• Institute for Electrical and Electronics
  Engineers (IEEE)

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Types of Data Transmission
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Parallel versus Serial Transmission
Parallel
Serial
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Asynchronous Communication

• Asynchronous and synchronous communications
• In broad sense, communication may be called
  asynchronous if transmitter and receiver do not
  explicitly coordinate before each data
  transmission
• Sender can wait arbitrarily long between
  transmissions. Sends when data becomes ready
• Used, for example, when sender may not always
  have data ready to send (E.g., keyboard, mouse)
• Receiver does not know when a character will
  arrive. May wait forever
• In more technical sense, Asynchronous may also
  mean no explicit information about where
  individual data bits begin and end

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Asynchronous Vs. Synchronous

• Synchronous
• May use a separate clock signal to indicate
  duration of bits.
• Asynchronous
• How do we know beginning of a bit and the ending
  of it?
• One way to ensure meaningful exchange
• Sender and receiver agree on bit duration
• Start bit before character
• One or more stop bits after character
• 1s when idle

e.g., 010100110100110101110011010111111111
Ch1 Ch2 Ch3 Idle
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Asynchronous Transmission
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The RS-232C Standard

• Standard specified by EIA
• For transfer of characters across copper wire
• Full name is RS-232-C common name is RS-232
• Defines serial, asynchronous communication
• Serial - bits are encoded and transmitted one at
  a time (as opposed to parallel transmission)
• Asynchronous - characters can be sent at any time
  and bits are not individually synchronized

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More Details about RS-232

• Components of standard
• Data represented by voltage between 15 and 15
• Cable limited to 50 feet
• 25-pin connector, with specific signals such as
  data, ground and control assigned to designated
  pins
• Specifies transmission of characters between,
  e.g., a terminal and a modem
• Transmitter never leaves wire at 0v when idle,
  transmitter puts negative voltage (a 1) on the
  wire

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RS-232 Character Transmission

• Transmitter indicates start of next character by
  transmitting a zero
• Receiver can detect transition as start of
  character
• Extra zero called the start bit
• Transmitter must leave wire idle so receiver can
  detect transition marking beginning of next
  character
• Transmitter sends a one after each character
• Extra one called the stop bit
• Thus, character represented by 7 data bits
  requires transmission of 9 bits across the wire

RS-232 terminology MARK is a negative voltage
( 1) SPACE is a positive voltage ( 0)
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Duration of a Bit

• Transmitter and receiver must agree on timing of
  each bit
• Agreement accomplished by choosing transmission
  rate
• Measured in bits per second
• Detection of start bit indicates to receiver when
  subsequent bits will arrive
• Hardware can usually be configured to select
  matching bit rates
• Switch settings
• Software
• Autodetection

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Bit Rate And Baud Rate

• Baud rate measures number of signal changes per
  second
• Bits per second measures number of bits
  transmitted per second
• In RS-232, each signal change represents one bit,
  so baud rate and bits per second are equal
• If each signal change represents more than one
  bit, bits per second may be greater than baud
  rate
• Bit rate Baud rate the number of bits
  represented by each signal unit
• Example An analog signal carries 4 bits in each
  signal change. If 1000 signal changes are sent
  per second, then baud rate 1000 bauds per
  second,
• bit rate 1000 4 4000 bps

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Character Framing

• Start and stop bits represent framing of each
  character
• If transmitter and receiver are using different
  speeds, stop bit will not be received at the
  expected time
• Problem is called a framing error
• RS-232 devices may send an intentional framing
  error called a BREAK
• E.g., ASCII keyboard BREAK key
• Deliberately create a framing error. Applications
  noticing a framing error as a request to abort

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Full-Duplex Communication

• Simultaneous two-way communication
• Requires each side to have transmitter and
  receiver
• Requires an electrical path in each direction
• Transmitter on one side connected to receiver on
  other
• Separate wires needed to carry current in each
  direction
• Common ground wire

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DB-25 Connection Standard

• RS-232 specifies use of 25 pin connector (DB-25)
• Pins are assigned for use as data, ground and
  control
• Pin 2 - Receive (RxD)
• Pin 3 - Transmit (TxD)
• Pin 4 - Ready to send (RTS)
• Pin 5 - Clear to send (CTS)
• Pin 7 Ground
• Commonly DB-9 is used.

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2-3 Swap

• Cable must cross-over wires to connect pins 2 and
  3 on receiver and transmitter
• RS-232 specifies that modems (DTE) transmit on
  pin 2 and receive on pin 3, while computers (DCE)
  transmit on pin 3 and receive on pin 2
• DTE Data Terminal Equipment (modem)
• DCE Data Computer Equipment (computer)
• To connect two DCEs (Computers), RS-232 cables
  between two computers must have 2-3 swap
• Also called null modem cable
• To connect DCE to DTE, use a straight through
  cable
• Standard modem cable

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Electric Transmission

• In real world
• Electric energy dissipates as it travels along
• Wires have resistance, capacitance, and
  inductance which distort signals
• Magnetic or electrical interference distorts
  signals
• Distortion can result in loss or
  misinterpretation

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Distorted Signal For A Single Bit

• In practice
• Distortion can be much worse than illustrated

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Consequences

• RS-232 hardware must handle minor distortions
• Take multiple samples per bit
• Tolerate less than full voltage
• Can not use electrical current for long-distance
  transmission
• Use carrier signals

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Newer Standards

• USB (Universal Serial Bus)
• Hot pluggable
• Device powered by bus
• Upto 12 Mbps
• Newer USB2 speed is 480 Mbps
• Firewire a high speed serial bus
• 400/800 Mbps

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Conclusions

• Asynchronous communication - data can start at
  any time individual bits not delineated
• RS-232 - EIA standard for asynchronous character
  transmission
• Bit rate and baud rate
• Bandwidth limits maximum data transmission rate
• Newer standards such as USB and Firewire are
  replacing the role of RS 232 in PCs