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Unformatted text preview: Connexions module: m12293 1 Serial Port Communication Version 1.1: Jul 16, 2004 3:39 pm GMT-5 National Instruments This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License * Abstract In this lesson, you will learn about serial communication. Serial Port Communication Serial communication (Figure 1) is a popular means of transmitting data between a computer and a peripheral device such as a programmable instrument or even another computer. Serial communication uses a transmitter to send data, one bit at a time, over a single communication line to a receiver. You can use this method when data transfer rates are low or you must transfer data over long distances. Serial communication is popular because most computers have one or more serial ports, so no extra hardware is needed other than a cable to connect the instrument to the computer or two computers together. Serial communication requires that you specify the following four parameters: • The baud rate of the transmission • The number of data bits encoding a character • The sense of the optional parity bit • The number of stop bits Each transmitted character is packaged in a character frame that consists of a single start bit followed by the data bits, the optional parity bit, and the stop bit or bits. Figure 2 shows a typical character frame encoding the letter m . Baud rate is a measure of how fast data are moving between instruments that use serial communication. RS-232 uses only two voltage states, called MARK and SPACE . In such a * http://creativecommons.org/licenses/by/1.0 Figure 1: 1: RS-232 Instrument,2: RS-232 Cable, 3: Serial Port http://cnx.org/content/m12293/latest/ Connexions module: m12293 2 Figure 2 two-state coding scheme, the baud rate is identical to the maximum number of bits of information, including control bits, that are transmitted per second. MARK is a negative voltage, and SPACE is positive. Figure 2 shows how the idealized signal looks on an oscilloscope. The following is the truth table for RS-232: Signal > 3 V = 0 Signal >- 3 V = 1 The output signal level usually swings between +12 V and -12 V. The dead area between +3 V and -3 V is designed to absorb line noise....
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- Spring '09
- Serial communication, Serial port, RS-232, output output output, serial port communication