Serial Communication – RS 232

January 31, 2011 — Leave a comment

Serial communication is basically the transmission or reception of data one bit at a time. Today’s computers generally address data in bytes or some multiple thereof.  A bit is basically either a logical 1 or zero.  Every character on this page is actually expressed internally as one byte.  The serial port is used to convert each byte to a stream of ones and zeros as well as to convert streams of ones and zeroes to bytes.  The serial port contains an electronic chip called a Universal Asynchronous Receiver/Transmitter (UART) that actually does the conversion.

Serial communication can be half duplex or full duplex. Full duplex communication means that a device can receive and transmit data at the same time. Half duplex means that the device cannot send and receive at the same time. It can do them both, but not at the same time. Half duplex communication is all but outdated except for a very small focused set of applications.
Half duplex serial communication needs at a minimum two wires, signal ground and the data line. Full duplex serial communication needs at a minimum three wires, signal ground, transmit data line, and receive data line. The RS232 specification governs the physical and electrical characteristics of serial communications. This specification defines several additional signals that are asserted (set to logical 1) for information and control beyond the data signals and signal ground.
These signals are the Carrier Detect Signal(CD), asserted by modems to signal a successful connection to another modem, Ring Indicator (RI), asserted by modems to signal the phone ringing, Data Set Ready (DSR), asserted by modems to show their presence, Clear To Send (CTS), asserted by modems if they can receive data, Data Terminal Ready(DTR), asserted by terminals to show their presence, Request To Send(RTS), asserted  by terminals if they can receive data. The section RS232 cabling describes these signals and how they are connected.

RS-232 Waveform

TTL/CMOS Serial Logic Waveform

The diagram above shows the expected waveform from the UART when using the common 8N1 format. 8N1 signifies 8 Data bits, No Parity and 1 Stop Bit. The RS-232 line, when idle is in the Mark State (Logic 1). A transmission starts with a start bit which is (Logic 0). Then each bit is sent down the line, one at a time. The LSB (Least Significant Bit) is sent first. A Stop Bit (Logic 1) is then appended to the signal to make up the transmission.

The data sent using this method, is said to be framed. That is the data is framed between a Start and Stop Bit.RS-232 Voltage levels1.     +3 to +25 volts to signify a “Space” (Logic 0) 2.  -3 to -25 volts for a “Mark” (logic 1). 3.  Any voltage in between these regions (i.e. between +3 and -3 Volts) is undefined.
The data byte is always transmitted least-significant-bit first.The bits are transmitted at specific time intervals determined by the baud rate of the serial signal.This is the signal present on the RS-232 Port of your computer, shown below.

RS-232 Logic Waveform



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