Serial Communication

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  • Serial communication is a form of I/O in which the bits of a byte begin transferred appear one after the other in a timed sequence on a single wire. 
  • Serial Communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. 
  • Serial communication is used for all long-haul communication and most computer networks, where the cost of cable and synchronization difficulties make parallel communication impractical. 
  • The serial port sends and receives bytes of information one bit at a time. Serial is used to transmit ASCII (American Standard Code for Information Interchange) data. Communication is completed using 3 transmission lines:
    • Ground
    • Transmit
    • Receive

  • Since serial is asynchronous, the port is able to transmit data on one line while receiving data on another.
  • Serial characteristics are baud rate, data bits, stop bits, and parity.
a) Baud rate 
  • Baud rate is a speed measurement for communication. 
  • It indicates the number of bit transfers per second. 
  • For example, 300 baud is 300 bits per second. 
  • When we refer to a clock cycle, we mean the baud rate. For example, if the protocol calls for a 4800 baud rate, then the clock is running at 4800 Hz. 
b) Data bits
  • Data bits is a measurement of the actual data bits in a transmission. 
  • When the computer sends a packet of information, the amount of actual data may not be a full 8 bits. 
  • Standard values for the data packets are 5, 7, and 8 bits. 
  • Which setting you choose depends on what information you are transferring. 
  • For example, standard ASCII has values from 0 to 127 (7 bits) 
  • Extended ASCII uses 0 to 255 (8 bits). 
  • If the data being transferred is simple text (standard ASCII), then sending 7 bits of data per packet is sufficient for communication. 
c) Stop bits
  • Stop bits is used to signal the end of communication for a single packet. 
  • Since the data is clocked across the lines and each device has its own clock, it is possible for the two devices to become slightly out of sync. 
  • The stop bits not only indicate the end of transmission but also give the computers some room for error in the clock speeds. 
d) Parity
  • Parity is a simple form of error checking that is used in serial communication. 
  • There are four types of parity: even, odd, marked, and spaced. 
  • The option of using no parity is also available.
    • Even parity
      • The parity bit would be 0 to keep the number of logic high bits even.
    • Odd parity
      • The parity bit would be 1 to keep the number of logic high bits odd.
  • Marked and spaced parity does not actually check the data bits, but simply sets the parity bit high for marked parity or low for spaced parity.
  • This allows the receiving device to know the state of a bit so as to enable the device to determine if noise is corrupting the data or if the transmitting and receiving devices' clocks are out of sync.



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