Chapter 5 Asynchronous Communication

1
Chapter 5 Asynchronous Communication

• Sender and receiver do not synchronize before
  data transmission
• Receiver does not know when individual bits begin
  and end

2
RS-232 Standard

• Used in async serial transmission
• Eg. Dumb terminals, terminal server, analog
  modems, console ports
• sender transmit character anytime
• arbitrarily delay between characters
• Binary 1 represented by 15 volts
• Binary 0 represented by 15 volts
• Idle voltage is 15 volts with minimum duration
  of one bit
• Start bit (binary 0) sent before character
• RS-232 voltage diagram (fig 5.2)

3
RS-232 (cont.)

• Baud rate
• number of changes in the signal per second.
• Bits per second may be equal to or greater than
  baud rate
• Supports full duplex
• DTE equipment (eg. computer) transmits on pin 2
  and receives on pin 3
• DCE equipment (eg. modem) transmits on pin 3 and
  receive on pin 2
• Minimum 3 wires (fig 5.3)
• DTR,DSR,DCD,RTS,CTS,TxD,RxD,GND
• RTS/CTS used in hardware flow control

4
Bandwidth

• maximum rate (Hertz) that a transmission system
  can change a signal
• Maximum theoretical data rate D (bits/s) for a
  transmission system with bandwidth B and uses K
  unique values of voltages
• D 2Blog 2 K (Nyquists theorem)