Serial Port Communication

LabVIEW 2018 Help

Edition Date: March 2018
Part Number: 371361R-01
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Serial communication transmits data between a computer and a peripheral device, such as a programmable instrument or another computer. Serial communication uses a transmitter to send data one bit at a time over a single communication line to a receiver. Use this method when data transfer rates are low or you must transfer data over long distances. Most computers have one or more serial ports, so you do not need any extra hardware other than a cable to connect the instrument to the computer or to connect two computers to each other.

You must specify four parameters for serial communication: the baud rate of the transmission, the number of data bits that encode a character, the sense of the optional parity bit, and the number of stop bits. A character frame packages each transmitted character as a single start bit followed by the data bits.

Baud rate is a measure of how fast data moves between instruments that use serial communication.

Data bits are transmitted upside down and backwards, which means that inverted logic is used and the order of transmission is from least significant bit (LSB) to most significant bit (MSB). To interpret the data bits in a character frame, you must read from right to left and read 1 for negative voltage and 0 for positive voltage.

An optional parity bit follows the data bits in the character frame. The parity bit, if present, also follows inverted logic. This bit is included as a means of error checking. You specify ahead of time for the parity of the transmission to be even or odd. If you choose for the parity to be odd, the parity bit is set in such a way so the number of 1s add up to make an odd number among the data bits and the parity bit.

The last part of a character frame consists of 1, 1.5, or 2 stop bits that are always represented by a negative voltage. If no further characters are transmitted, the line stays in the negative (MARK) condition. The transmission of the next character frame, if any, begins with a start bit of positive (SPACE) voltage.

Rate of Data Transfer

You can calculate the maximum transmission rate in characters per second for a given communication setting by dividing the baud rate by the bits per character frame.

Serial Hardware Overview

The following examples are the most common recommended standards of serial port communication:

  • RS232 (ANSI/EIA-232 Standard) is used for many purposes, such as connecting a mouse, printer, or modem. It also is used with industrial instrumentation. Because of improvements in line drivers and cables, applications often increase the performance of RS232 beyond the distance and speed in the standards list. RS232 is limited to point-to-point connections between PC serial ports and devices.
  • RS422 (AIA RS422A Standard) uses a differential electrical signal as opposed to the unbalanced (single-ended) signals referenced to ground with RS232. Differential transmission, which uses two lines each to transmit and receive signals, results in greater noise immunity and longer transmission distances as compared to RS232.
  • RS485 (EIA-485 Standard) is a variation of RS422 that allows you to connect up to 32 devices to a single port and define the necessary electrical characteristics to ensure adequate signal voltages under maximum load. With this enhanced multidrop capability, you can create networks of devices connected to a single RS485 serial port. The noise immunity and multidrop capability make RS485 an attractive choice in industrial applications that require many distributed devices networked to a PC or other controller for data collection and other operations.

Connecting a Serial Device

If you have a serial device in a system, you first must obtain the pinout for that device and make sure you have the correct cable to connect the serial device to the computer. Determine if the device is data communications equipment (DCE) or data terminal equipment (DTE) and what settings it uses to communicate—baud rate, data bits, stop bits, parity, or handshaking (flow control).

More Information About Serial Communication

Refer to the NI Developer Zone for more information on communicating with Serial instruments.


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