Serial Communication

• 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.