05-07-2013, 03:52 PM
Introduction to serial communications
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Introduction
The serial communications are used for transferring data over long distances, because parallel communications requires too many wires. Serial data received from a modem or other devices are converted to parallel so that it can be transferred to the PC bus.
The serial communications equipment can be divided into simplex, half-duplex and full-duplex.
A simplex serial communication sends information only in one direction (i.e. a commercial radio station).
Half-duplex means that data can be send in either direction between two systems, but only in one direction at a time.
In a full-duplex transmission each system can send and receive data at the same time.
There are two ways to transmit serial data: synchronously or asynchronously. In a synchronous transmission data is sent in blocks, the transmitter and the receiver are synchronized by one or more special characters called sync characters.
The serial port of the PC is an asynchronous device, so we will describe this kind of systems. For asynchronous transmission, a bit identifies its start and 1 or 2 bits identify its end, don't need any synchronization. The data bits are sent to the receiver after the start bit. The least significant bit is transmitted first. A data character usually consists of 7 or 8 bits. Depending on the configuration of the transmission a parity bit is send after each data bit. It is used to check errors in the data characters. Finally 1 or 2 stop bits are send.
RS232 serial Communication
Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. Independent channels are established for two-way (full-duplex) communications. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol.
The RS-232 interface presupposes a common ground between the DTE and DCE. This is a reasonable assumption when a short cable connects the DTE to the DCE, but with longer lines and connections between devices that may be on different electrical busses with different grounds, this may not be true.
RS232 data is bi-polar.. +3 TO +12 volts indicate an "ON or 0-state (SPACE) condition" while A -3 to -12 volts indicates an "OFF" 1-state (MARK) condition.. Modern computer equipment ignores the negative level and accepts a zero voltage level as the "OFF" state. In fact, the "ON" state may be achieved with lesser positive potential. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced.
PIN SIGNAL DESCRIPTION
1 PGND Protective Ground
2 TXD Transmit Data
3 RXD Receive Data
4 RTS Ready To Send
5 CTS Clear To Send
6 DSR Data Set Ready
7 SG Signal Ground
8 CD Carrier Detect
20 DTR Data Terminal Ready
22 RI Ring Indicators