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Monday, 9 December 2019

RS232 Parameters, Start and Stop Bits

RS232

RS232 (Recommended Standard 232) is the standard developed by EIA (Electronic Industry Association) in the year 1960 for the serial communications of data. This standard is a widely used standard, the PC consist of two RS232 ports Com1 and Com2.  Rs232 is used to connect a DTE (Data Terminal Equipment) to a DCE (Data Communications Equipment). For E.g. ., DTE is pc, printer and DCE is Physical interface. In RS232, data are transmitted serially and Provides full duplex Communication.


RS232 Parameters

When two devices want to communicate, the sender sends data as character by Character. The character is corresponds to a bit. The number of characters is called data bits. The data is appended with one start bit at prefix and stop bit in the suffix. The receiver decodes the data using start and stops bits and receives it. This type of communication is called an asynchoronous communication, because no clock is used. Parity bit is also appended to the data to be sent in order to achieve error correction. For two devices to communicate through RS232 the communication parameters should set in both systems. They are Data bits, data rate, start bit, stop bit, parity bit, flow control.

Data rate

• Data rate is the rate at which the data communication takes place.

• The PC supports various data rates such as, 50,150,300,600,1200,2400,4800,19200,38400,57600 and 115200 bps.

• The oscillator in the RS232 circuitry operates at 1.845 MHZ.

• It is divided by 1600 to obtain 115200 data.

Data bits

• Data bits is the Number of of bits transmitted for each character.
• The character can have 5 or 6 or 7 or 8 bits.
• If ASCII character is send, the number of bits is 7.

Start bits

• Start bit is the bit that is prefixed to the data in order to identify the beginning of the character.

Stop bits

• These bits are appended to the data bits to identify the end of character.
• If the data bits are 7 or 8, one stop bit is appended.
• If the data bits are 5 or 6, two stop bits are appended.

Parity

• Parity bits are appended to the character for error checking.
• The parity can be even or odd.
• For even parity, the parity will make the total no. of bits even.
• For odd parity, the parity will make the total no. of bits odd.

Flow control

• Flow control can be defined as a protocol to stop/resume data transmission. It is also known as hand shaking protocol.

• For Hardware handshaking, two signals are used:

• RTS-REQUEST TO SEND

• CTS-Clear To Send

• The software handshaking is known as XON/XOFF.

RS232 Connector Configurations

• RS232 specifies two types of connectors:

25-pin connector
9-pin connector

• Only few pins are used in 25-pin connector.

• Important pins are 2 (Transmit), 3 (Receive) 7 (Signal Ground)

• RS232 uses unbalanced transmission and is susceptible to noise.


For the 25-pin Connector

Pin Number
Function (Abbreviation)
1
Chassis Ground
2
Transmit Data (TXD)
3
Receive Data (RXD)
4
Request To Send (RTS)
5
Clear To Send (CTS)
6
Data Set Ready (DSR)
7
Signal Ground (GND)
8
Carrier Detect (CD)
20
Data Terminal Ready (DTR)
22
Ring Indicator (RI)

For the 9-pin Connector

Pin Number
Function (Abbreviation)
1
Carrier Detect (CD)
2
Receive Data (RXD)
3
Transmit Data (TXD)
4
Data Terminal Ready (DTR)
5
Signal Ground (GND)
6
Data Set Ready (DSR)
7
Request To Send (RTS)
8
Clear To Send (CTS)
9
Ring Indicator (RI)



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