Communication Serial Data


Communication is the method to share information, ideas or opinions. It is a two-way action between two or more people or devices. It is defined as follows

“Communication is the process of transferring information from a sender to a receiver with the use of a medium in which the communicated information is understood by both sender and receiver”.[5]

Communication allows collaborating and cooperating with each other. So in order to perform successful communication it is necessary that both the parties understand a common language so that the data can be exchanged.

4.1Communication Process

Figure 4-1: Communication Process

4.2Communication Networks

In a digital communications system, there are two methods for data transfer:

  • Parallel
  • Serial

4.2.1Parallel Communication

In Parallel communication data is transmitted over a communication link simultaneously. All the bits of each symbol which are to be sent are transmitted together. A parallel port usually sends and receives data eight bits at a time over 8 separate wires. This method of data transfer is quick but using large number of wires simultaneously is a problem. Connecting a PC to a printer is the example where parallel port is used.

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4.2.2 Serial Communication

Serial communication is the process of sending data one by one. Each bit of symbol is sent one bit at one time, sequentially, over a communication channel .A serial port sends and receives data one bit at a time over one wire. Although the time require by serial communication is much more than that in parallel communication, but only a few wires are required to send data successfully. Serial communications include most network devices, keyboards, mice, Modems, and terminals.

4.2.3 Difference between Parallel and Serial

In order to establish a communication link between two devices there may be serial or parallel communication. A parallel link transmits several streams of along communication channel whereas a serial link transmits a single stream of data.

It seems that parallel communication is much better and efficient than serial communication. But normally serial communication is more common in use as it is cheaper to implement and fewer wires are required to send data. Furthermore, serial communication allows sending data over longer distances.

4.3Description of Serial Communication

Serial communication is the process of transmitting data between a computer and a peripheral device such as a programmable instrument or even another computer.

Serial communication is used for all long-haul communication and most computer networks, where the cost of cable and synchronization difficulties makes parallel communication impractical. Serial computer buses are becoming more common as improved technology enables them to transfer data at higher speeds.


In order to transmit data between two devices it must be necessary that the data must be synchronized. Synchronization needs to be done so that the receiving system have an idea from where the first byte starts and where it ends, Methods of Synchronization

There are two ways to synchronize the two ends of the communication.

  • Synchronous transmission
  • Asynchronous transmission Synchronous Transmission

In synchronous transmission a continuous stream of data is sent at regular intervals. A clock is used which synchronize the receiver with the transmitter. The data is transferred in the form of packets. Each packet contains

  • Start frame
  • Stop frame
  • Data

The function of start frame is to indicate the arrival of a new packet and secondly synchronized with the receiver clock. The function of end frame is to indicate that the end bits of the message when the data ends. Data is present between the start frame and the stop frame. The maximum bits of data in a packet are 64,000 bits.

End Frame

Data (Max 8K bytes)

Start Frame

There are two methods of synchronous transmission.

  • Data/strobe synchronous transmission

In this type of transmission stream of data is put on one wire and pulse of voltage on other wire (strobe). This tells the receiver about the start and stop bits.

  • Single wire synchronous transmission
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In this type of transmission a single wire is used to transmit the signal. Manchester code is the example of this transmission. One is indicated when signal shifted from a low state to a high state and zero is indicated on switching from high to low state.

The major advantage of synchronous transmission is that fewer overheads are required to send data with greater efficiency and less chance of loss of synchronization. But at the same time it is more difficult to implement and is also costly. Asynchronous Transmission

In asynchronous transmission, signal is transmitted on a signal wire. The periodic stream of data is transmitted by the transmitter and is distinguished by the receiver as a sync signal.

In asynchronous transmission seven or eight bit of data are transmitted in the form of ASCII characters. The character specifies the start and stop bits. E.g. “1000 0101” is transmitted as “1 1000 0101 0” with 1 as start bit and 0 as stop bit. A parity bit can also be included in the data stream.











Stop bit

Start bit

4.4Universal Asynchronous Receiver/Transmitter

A Universal Asynchronous receiver/transmitter (UART) is a type of asynchronous transmitter receiver. It translates the data in serial and parallel form and is commonly used in microcontrollers.

For serial communication following features are required:

  • Baud rate
  • Start bits
  • Data bits
  • Parity bits

Character Frame





Stop Bits

Parity Bits

Data Bits

Start Bits

  • Stop bits

Figure 4-1: Character frame encoding

4.4.1Baud Rate

The baud rate of a transmission is how many symbols are transferred in a second. There are two voltage stages MARK and SPACE. MARK represents the negative voltage and SPACE represents the positive voltage.

4.4.2Start Bits

Start bit informs about the initialization of a new frame of data. The duration of start bit is the reciprocal of the baud rate. If Baud rate of the transmission is 9600 then the duration of start bit will be 0.104ms and the entire character frame of 11 bits would be transmitted in 1.146ms.

4.4.3Data Bits

Data bits represent data in a character frame. The order of transmission is from LSB to MSB.

4.4.4Parity Bits

It is an optional bit in the data frame. It is transmitted after the data bits. It helps in error checking and detection of data corruption. There are different ways of parity checking i.e. even parity or odd parity. The number of marks is counted in a data frame and if the number is even a single bit is used to represent that the parity is even similarly if the number of logic 1 is odd a single bit represents that the parity of the data frame is odd. For the case of even parity the bit is 1 and for the case of odd parity the bit is 0.

4.4.5Stop Bits

In a character frame stop bits is the last frame. Stop bits are represented by a negative voltage (SPACE). The stop bit stays at the MARK (negative position) is no further data is transmitted. Otherwise a start bit of SPACE (positive voltage) is transmitted.

4.6AT Commands

AT is the Attention command. This command instructs the modem to perform many communication tasks e.g. dial, answer hook control etc. All the modem commands precedes this attention command except A/ and +++.


This command is used to check the mode of operation of the mobile. There are two modes for mobile communication

  • Text mode
  • PDU mode (Protocol Data Unit)

AT + CMGF = 1 refers to PDU mode whereas AT+CMGF = 0 refers to text PDU mode.

AT + CMGF =? refers to the mode which is in used after execution.


The command reads the message from a desired memory location. E.g. if AT + CMGR = 1 then it will read the message from first memory location.

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The command deletes the message from a desired memory location .E.g AT+CMGD = 0 deletes the message from index 0.


This command is used to read, write, delete, send and receive from memory storage.

AT+CPMS = "ME" refers to message storage on mobile phone. AT+CPMS = "SIM” refers to message storage on SIM.


The command gives the indication of new message. AT+CNMI = 2,1,2,0,0

AT+CMGS ="Any mobile number"

This command confirms that either switching on or off of device is performed or not. This command sends message to the phone network. On successful delivery, a message reference number is returned.