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RS-232 serial interface


RS232 interface DB9

RS232 is the interface mainly used for serial data communication. It supports data transfer rate from about 110bps to about 115200 bps (bits per sec). Hyper terminal is the application mainly used to check serial communication port of the computer, often referred as COM port.

The interface is two types DB9 and DB25 pin connectors. The interface is mainly used for one to one serial communication for example computer to dial up modem connection. Figure-1 depicts RS232 pin diagrams for DB9 connector.




RS232 interface DB25

Figure 2 depicts RS232 pin diagrams for DB25 connector. As shown in the figure, it carries both data and control signal lines. These are mentioned below.

RD- Receive data, it receives data from DCE (Data communication Equipment) and pass to DTE (Data Terminal Equipment). For example, DTE is the computer and DCE is the modem connected to the telephone line.
TD- Transmit data, it carries data from DTE to DCE.
CD- Carrier Detect, signal from DCE to DTE. Indicates that modem (DCE) is busy i.e. already using the line.
RI- Signal from DCE to DTE, Modem (DCE) detects ring from line.
RTS- Request to Send, DTE requests DCE to become ready to accept the data.
CTS- Clear to Send, DCE acknowledges DTE that I am ready to receive the data. (Response of RTS).
DTR- Data Terminal Ready, DTE is ready to transmit.
DSR- Data Set Ready, DCE is ready to receive.
SGND- Signal Ground

RS232 protocol

Following are the RS232 protocol details.
• Binary 0 range from about +5 to +15 voltage (DC)
• Binary 1 range from about -5 to -15 voltage (DC)
• Start bit - Binary 0
• Data- 5, 6, 7 or 8 bit configurable
• Parity- Odd/even, is not used with 8 bit of data.
• Stop bit: Binary 1, one or two bits

RS232 technical specifications

The table-1 below mentions RS232 interface technical specifications.


Specifications RS232
Mode of operation Single ended
number of drivers/receivers on one line 1 driver, 1 receiver
maximum cable length 50 feet
maximum data rate 460kbps
max. driver output voltage +/- 25 V
Max. driver current in high impedance state +/-6mA at +/-2V
slew rate(max) 30 V/µS
receiver input voltage range +/-15V
Receiver input sensitivity +/-3V
Driver load impedance 3K to 7K Ohms
Receiver Input Resistance 3K to 7K Ohms

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