llowing features are required:
Baud rate
Start bits
Data bits
Parity bits
Character Frame
Idle
Space
Mark
Idle
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 +++.
AT+CMGF
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.
AT+CMGR
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.
AT+CMGD
The command deletes the message from a desired memory location .E.g AT+CMGD = 0 deletes the message from index 0.
AT+CPMS
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.
AT+CNMI
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 ph
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