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Serial Port Communication

Module by: National Instruments

Summary: In this lesson, you will learn about serial communication.

Serial communication 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.
Figure 1: 1: RS-232 Instrument,2: RS-232 Cable, 3: Serial Port
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.
Figure 2
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 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>3V=0 Signal 3 V 0 Signal>-3V=1 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.
A start bit signals the beginning of each character frame. It is a transition from negative (MARK) to positive (SPACE) voltage. Its duration in seconds is the reciprocal of the baud rate. If the instrument is transmitting at 9,600 baud, the duration of the start bit and each subsequent bit is about 0.104 ms. The entire character frame of eleven bits would be transmitted in about 1.146 ms.
Data bits are transmitted upside down and backwards. That is, 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. This yields 1101101 (binary) or 6D (hex). An ASCII conversion table shows that this is the letter m.
An optional parity bit follows the data bits in the character frame. The parity bit, if present, also follows inverted logic, 1 for negative voltage and 0 for positive voltage. This bit is included as a simple means of error handling. You specify ahead of time whether the parity of the transmission is to be even or odd. If the parity is chosen to be odd, the transmitter then sets the parity bit in such a way as to make an odd number of ones among the data bits and the parity bit. This transmission uses odd parity. There are five ones among the data bits, already an odd number, so the parity bit is set to 0.
The last part of a character frame consists of 1, 1.5, or 2 stop bits. These bits 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, is heralded by a start bit of positive (SPACE) voltage.

How Fast Can I Transmit?

Knowing the structure of a character frame and the meaning of baud rate as it applies to serial communication, you can calculate the maximum transmission rate, in characters per second, for a given communication setting. This rate is just the baud rate divided by the bits per frame. In the previous example, there are a total of eleven bits per character frame. If the transmission rate is set at 9,600 baud, you get 9,60011=872 9,600 11 872 characters per second. Notice that this is the maximum character transmission rate. The hardware on one end or the other of the serial link might not be able to reach these rates, for various reasons.

Hardware Overview

There are many different recommended standards of serial port communication, including the following most common types.

RS-232

The RS-232 is a standard developed by the Electronic Industries Association (EIA) and other interested parties, specifying the serial interface between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE). The RS-232 standard includes electrical signal characteristics (voltage levels), interface mechanical characteristics (connectors), functional description of interchange circuits (the function of each electrical signal), and some recipes for common kinds of terminal-to-modem connections. The most frequently encountered revision of this standard is called RS-232C. Parts of this standard have been adopted (with various degrees of fidelity) for use in serial communications between computers and printers, modems, and other equipment. The serial ports on standard IBM-compatible personal computers follow RS-232.

RS-449, RS-422, RS-423

The RS-449, RS-422, and RS-423 are additional EIA serial communication standards related to RS-232. RS-449 was issued in 1975 and was supposed to supersede RS-232, but few manufacturers have embraced the newer standard. RS-449 contains two subspecifications called RS-422 and RS-423. While RS-232 modulates a signal with respect to a common ground, or single-ended transmission, RS-422 modulates two signals against each other, or differential transmission. The RS-232C receiver senses whether the received signal is sufficiently negative with respect to ground to be a logical 1, whereas the RS-422 receiver senses which line is more negative than the other. This makes RS-422 more immune to noise and interference and more versatile over longer distances. The Macintosh serial ports follow RS-422, which can be converted to RS-423 by proper wiring of an external cable. RS-423 can then communicate with most RS-232 devices over distances of 15 m or so.

RS-232 Cabling

Devices that use serial cables for their communication are split into two categories. These are DCE and DTE. DCE are devices such as a modem, TA adapter, plotter, and so on, while DTE is a computer or terminal. RS-232 serial ports come in two sizes, the D-Type 25-pin connector and the D-Type 9-pin connector. Both of these connectors are male on the back of the PC. Thus, you require a female connector on the device. Table 1 shows the pin connections for the 9-pin and 25-pin D-Type connectors.
Figure 3
Function Signal PIN DTE DCE
Data TxD 3 Output Input
RxD 2 Input Output
Handshake RTS 7 Output Input
CTS 8 Input Output
DSR 6 Input Output
DCD 1 Input Output
STR 4 Output Input
Common Com 5 -- --
Other RI 9 Output Input
The DB-9 connector is occasionally found on smaller RS-232 lab equipment. It is compact, yet has enough pins for the core set of serial pins (with one pin extra).
Note: The DB-9 pin numbers for transmit and receive (3 and 2) are opposite of those on the DB-25 connector (2 and 3). Be careful of this difference when you are determining if a device is DTE or DCE.
The DB-25 connector is the standard RS-232 connector, with enough pins to cover all the signals specified in the standard. Table 2 shows only the core set of pins that are used for most RS-232 interfaces.
Figure 4
Function Signal PIN DTE DCE
Data TxD 2 Output Input
RxD 3 Input Output
Handshake RTS 4 Output Input
CTS 5 Input Output
DSR 6 Input Output
DCD 8 Input Output
STR 20 Output Input
Common Com 7 -- --

Software Overview

Use the VIs and functions located on the Functions>>All Functions>>Instrument I/O>>Serial palette for serial port communication.
You used some of the VISA functions on this palette for GPIB communication. The VISA Write and VISA Read functions work with any type of instrument communication and are the same whether you are doing GPIB or serial communication. However, because serial communication requires you to configure extra parameters, you must start the serial port communication with the VISA Configure Serial Port VI.
The VISA Configure Serial Port VI initializes the port identified by VISA resource name to the specified settings. timeout sets the timeout value for the serial communication. baud rate, data bits, parity, and flow control specify those specific serial port parameters. The error in and error out clusters maintain the error conditions for this VI.
Example 1 
Figure 5 shows how to send the identification query command *IDN? to the instrument connected to the COM2 serial port. The VISA Configure Serial Port VI opens communication with COM2 and sets it to 9,600 baud, eight data bits, odd parity, one stop bit, and XON/XOFF software handshaking. Then the VISA Write function sends the command. The VISA Read function reads back up to 200 bytes into the read buffer, and the Simple Error Handler VI checks the error condition.
Figure 5
Note: The VIs and functions located on the Functions>>All Functions>>Instrument I/O>>Serial palette are also used for parallel port communication. You specify the VISA resource name as being one of the LPT ports. For example, you can use MAX to determine that LPT1 has a VISA resource name of ASRL10::INSTR.

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