Contents

Overview of the Serial Port

Introduction

For many serial port applications, you can communicate with your device without detailed knowledge of how the serial port works. If your application is straightforward, or if you are already familiar with the previously mentioned topics, you might want to begin with The Serial Port Session to see how to use your serial port device with MATLAB® software.

What Is Serial Communication?

Serial communication is the most common low-level protocol for communicating between two or more devices. Normally, one device is a computer, while the other device can be a modem, a printer, another computer, or a scientific instrument such as an oscilloscope or a function generator.

As the name suggests, the serial port sends and receives bytes of information in a serial fashion — one bit at a time. These bytes are transmitted using either a binary (numerical) format or a text format.

The Serial Port Interface Standard

The serial port interface for connecting two devices is specified by the TIA/EIA-232C standard published by the Telecommunications Industry Association.

The original serial port interface standard was given by RS-232, which stands for Recommended Standard number 232. The term RS-232 is still in popular use, and is used in this guide when referring to a serial communication port that follows the TIA/EIA-232 standard. RS-232 defines these serial port characteristics:

  • The maximum bit transfer rate and cable length

  • The names, electrical characteristics, and functions of signals

  • The mechanical connections and pin assignments

Primary communication is accomplished using three pins: the Transmit Data pin, the Receive Data pin, and the Ground pin. Other pins are available for data flow control, but are not required.

Other standards such as RS-485 define additional functionality such as higher bit transfer rates, longer cable lengths, and connections to as many as 256 devices.

Connecting Two Devices with a Serial Cable

The RS-232 standard defines the two devices connected with a serial cable as the Data Terminal Equipment (DTE) and Data Circuit-Terminating Equipment (DCE). This terminology reflects the RS-232 origin as a standard for communication between a computer terminal and a modem.

Throughout this guide, your computer is considered a DTE, while peripheral devices such as modems and printers are considered DCEs. Many scientific instruments function as DTEs.

Because RS-232 mainly involves connecting a DTE to a DCE, the pin assignments are defined such that straight-through cabling is used, where pin 1 is connected to pin 1, pin 2 is connected to pin 2, and so on. The following diagram shows a DTE to DCE serial connection using the transmit data (TD) pin and the receive data (RD) pin.

For more information about serial port pins, see Serial Port Signals and Pin Assignments.

If you connect two DTEs or two DCEs using a straight serial cable, the TD pins on each device are connected to each other, and the RD pins on each device are connected to each other. Therefore, to connect two like devices, you must use a null modem cable. As shown in the following diagram, null modem cables cross the transmit and receive lines in the cable.

    Note:   You can connect multiple RS-422 or RS-485 devices to a serial port. If you have an RS-232/RS-485 adaptor, you can use the MATLAB serial port object with these devices.

Serial Port Signals and Pin Assignments

Serial ports consist of two signal types: data signals and control signals. To support these signal types, as well as the signal ground, the RS-232 standard defines a 25-pin connection. However, most Windows® and UNIX® platforms use a 9-pin connection. In fact, only three pins are required for serial port communications: one for receiving data, one for transmitting data, and one for the signal ground.

The following diagram shows the pin assignment scheme for a 9-pin male connector on a DTE.

The pins and signals associated with the 9-pin connector are described in the following table. Refer to the RS-232 standard for a description of the signals and pin assignments used for a 25-pin connector.

Serial Port Pin and Signal Assignments

PinLabelSignal NameSignal Type

1

CD

Carrier Detect

Control

2

RD

Received Data

Data

3

TD

Transmitted Data

Data

4

DTR

Data Terminal Ready

Control

5

GND

Signal Ground

Ground

6

DSR

Data Set Ready

Control

7

RTS

Request to Send

Control

8

CTS

Clear to Send

Control

9

RI

Ring Indicator

Control

The term data set is synonymous with modem or device, while the term data terminal is synonymous with computer.

    Note:   The serial port pin and signal assignments are with respect to the DTE. For example, data is transmitted from the TD pin of the DTE to the RD pin of the DCE.

Signal States

Signals can be in either an active state or an inactive state. An active state corresponds to the binary value 1, while an inactive state corresponds to the binary value 0. An active signal state is often described as logic 1, on, true, or a mark. An inactive signal state is often described as logic 0, off, false, or a space.

For data signals, the on state occurs when the received signal voltage is more negative than -3 volts, while the off state occurs for voltages more positive than 3 volts. For control signals, the on state occurs when the received signal voltage is more positive than 3 volts, while the off state occurs for voltages more negative than -3 volts. The voltage between -3 volts and +3 volts is considered a transition region, and the signal state is undefined.

To bring the signal to the on state, the controlling device unasserts (or lowers) the value for data pins and asserts (or raises) the value for control pins. Conversely, to bring the signal to the off state, the controlling device asserts the value for data pins and unasserts the value for control pins.

The following diagram shows the on and off states for a data signal and for a control signal.

The Data Pins

Most serial port devices support full-duplex communication meaning that they can send and receive data at the same time. Therefore, separate pins are used for transmitting and receiving data. For these devices, the TD, RD, and GND pins are used. However, some types of serial port devices support only one-way or half-duplex communications. For these devices, only the TD and GND pins are used. This guide assumes that a full-duplex serial port is connected to your device.

The TD pin carries data transmitted by a DTE to a DCE. The RD pin carries data that is received by a DTE from a DCE.

The Control Pins

The control pins of a 9-pin serial port are used to determine the presence of connected devices and control the flow of data. The control pins include

The RTS and CTS Pins.  The RTS and CTS pins are used to signal whether the devices are ready to send or receive data. This type of data flow control—called hardware handshaking—is used to prevent data loss during transmission. When enabled for both the DTE and DCE, hardware handshaking using RTS and CTS follows these steps:

  1. The DTE asserts the RTS pin to instruct the DCE that it is ready to receive data.

  2. The DCE asserts the CTS pin indicating that it is clear to send data over the TD pin. If data can no longer be sent, the CTS pin is unasserted.

  3. The data is transmitted to the DTE over the TD pin. If data can no longer be accepted, the RTS pin is unasserted by the DTE and the data transmission is stopped.

To enable hardware handshaking in MATLAB software, see Controlling the Flow of Data: Handshaking.

The DTR and DSR Pins.  Many devices use the DSR and DTR pins to signal if they are connected and powered. Signaling the presence of connected devices using DTR and DSR follows these steps:

  1. The DTE asserts the DTR pin to request that the DCE connect to the communication line.

  2. The DCE asserts the DSR pin to indicate it is connected.

  3. DCE unasserts the DSR pin when it is disconnected from the communication line.

The DTR and DSR pins were originally designed to provide an alternative method of hardware handshaking. However, the RTS and CTS pins are usually used in this way, and not the DSR and DTR pins. Refer to your device documentation to determine its specific pin behavior.

The CD and RI Pins.  The CD and RI pins are typically used to indicate the presence of certain signals during modem-modem connections.

A modem uses a CD pin to signal that it has made a connection with another modem, or has detected a carrier tone. CD is asserted when the DCE is receiving a signal of a suitable frequency. CD is unasserted if the DCE is not receiving a suitable signal.

The RI pin is used to indicate the presence of an audible ringing signal. RI is asserted when the DCE is receiving a ringing signal. RI is unasserted when the DCE is not receiving a ringing signal (e.g., it is between rings).

Serial Data Format

The serial data format includes one start bit, between five and eight data bits, and one stop bit. A parity bit and an additional stop bit might be included in the format as well. The following diagram illustrates the serial data format.

The following notation expresses the format for serial port data:

number of data bits - parity type - number of stop bits

For example, 8-N-1 is interpreted as eight data bits, no parity bit, and one stop bit, while 7-E-2 is interpreted as seven data bits, even parity, and two stop bits.

The data bits are often referred to as a character because these bits usually represent an ASCII character. The remaining bits are called framing bits because they frame the data bits.

Bytes Versus Values

A byte is the collection of bits that comprise the serial data format. At first, this term might seem inaccurate because a byte is 8 bits and the serial data format can range between 7 bits and 12 bits. However, when serial data is stored on your computer, the framing bits are stripped away, and only the data bits are retained. Moreover, eight data bits are always used regardless of the number of data bits specified for transmission, with the unused bits assigned a value of 0.

When reading or writing data, you might need to specify a value, which can consist of one or more bytes. For example, if you read one value from a device using the int32 format, that value consists of four bytes. For more information about reading and writing values, see Writing and Reading Data.

Synchronous and Asynchronous Communication

The RS-232 standard supports two types of communication protocols: synchronous and asynchronous.

Using the synchronous protocol, all transmitted bits are synchronized to a common clock signal. The two devices initially synchronize themselves to each other, and continually send characters to stay synchronized. Even when actual data is not really being sent, a constant flow of bits allows each device to know where the other is at any given time. That is, each bit that is sent is either actual data or an idle character. Synchronous communications allows faster data transfer rates than asynchronous methods, because additional bits to mark the beginning and end of each data byte are not required.

Using the asynchronous protocol, each device uses its own internal clock, resulting in bytes that are transferred at arbitrary times. So, instead of using time as a way to synchronize the bits, the data format is used.

In particular, the data transmission is synchronized using the start bit of the word, while one or more stop bits indicate the end of the word. The requirement to send these additional bits causes asynchronous communications to be slightly slower than synchronous. However, it has the advantage that the processor does not have to deal with the additional idle characters. Most serial ports operate asynchronously.

    Note:   When used in this guide, the terms synchronous and asynchronous refer to whether read or write operations block access to the MATLAB command line. For more information, see Controlling Access to the MATLAB Command Line.

How Are the Bits Transmitted?

By definition, serial data is transmitted one bit at a time. The order in which the bits are transmitted is:

  1. The start bit is transmitted with a value of 0.

  2. The data bits are transmitted. The first data bit corresponds to the least significant bit (LSB), while the last data bit corresponds to the most significant bit (MSB).

  3. The parity bit (if defined) is transmitted.

  4. One or two stop bits are transmitted, each with a value of 1.

The baud rate is the number of bits transferred per second. The transferred bits include the start bit, the data bits, the parity bit (if defined), and the stop bits.

Start and Stop Bits

As described in Synchronous and Asynchronous Communication, most serial ports operate asynchronously. This means that the transmitted byte must be identified by start and stop bits. The start bit indicates when the data byte is about to begin; the stop bit(s) indicate(s) when the data byte has been transferred. The process of identifying bytes with the serial data format follows these steps:

  1. When a serial port pin is idle (not transmitting data), it is in an on state.

  2. When data is about to be transmitted, the serial port pin switches to an off state due to the start bit.

  3. The serial port pin switches back to an on state due to the stop bit(s). This indicates the end of the byte.

Data Bits

The data bits transferred through a serial port might represent device commands, sensor readings, error messages, and so on. The data can be transferred as either binary data or ASCII data.

Most serial ports use between five and eight data bits. Binary data is typically transmitted as eight bits. Text-based data is transmitted as either seven bits or eight bits. If the data is based on the ASCII character set, a minimum of seven bits is required because there are 27 or 128 distinct characters. If an eighth bit is used, it must have a value of 0. If the data is based on the extended ASCII character set, eight bits must be used because there are 28 or 256 distinct characters.

The Parity Bit

The parity bit provides simple error (parity) checking for the transmitted data. The following table shows the types of parity checking.

Parity Types

Parity TypeDescription

Even

The data bits plus the parity bit result in an even number of 1s.

Mark

The parity bit is always 1.

Odd

The data bits plus the parity bit result in an odd number of 1s.

Space

The parity bit is always 0.

Mark and space parity checking are seldom used because they offer minimal error detection. You might choose to not use parity checking at all.

The parity checking process follows these steps:

  1. The transmitting device sets the parity bit to 0 or to 1, depending on the data bit values and the type of parity-checking selected.

  2. The receiving device checks if the parity bit is consistent with the transmitted data. If it is, the data bits are accepted. If it is not, an error is returned.

    Note:   Parity checking can detect only 1-bit errors. Multiple-bit errors can appear as valid data.

For example, suppose the data bits 01110001 are transmitted to your computer. If even parity is selected, the parity bit is set to 0 by the transmitting device to produce an even number of 1s. If odd parity is selected, the parity bit is set to 1 by the transmitting device to produce an odd number of 1s.

Finding Serial Port Information for Your Platform

This section describes the ways to find serial port information for Windows and UNIX platforms.

    Note:   Your operating system provides default values for all serial port settings. However, these settings are overridden by your MATLAB code, and will have no effect on your serial port application.

Microsoft Windows Platform

You can access serial port information through the System Properties dialog. To access this on a Windows XP platform,

  1. Right-click My Computer on the desktop, and select Properties.

  2. In the System Properties dialog, click the Hardware tab.

  3. Click Device Manager.

  4. In the Device Manager dialog, expand the Ports node.

  5. Double-click the Communications Port (COM1) node.

  6. Select the Port Settings tab.

MATLAB displays the following Ports dialog box.

UNIX Platform

To find serial port information for UNIX platforms, you need to know the serial port names. These names might vary between different operating systems.

On a Linux® platform, serial port devices are typically named ttyS0, ttyS1, etc. Use the setserial command to display or configure serial port information. For example, to display which ports are available:

setserial -bg /dev/ttyS*
/dev/ttyS0 at 0x03f8 (irq = 4) is a 16550A
/dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A

To display detailed information about ttyS0:

setserial -ag /dev/ttyS0
/dev/ttyS0, Line 0, UART: 16550A, Port: 0x03f8, IRQ: 4
        Baud_base: 115200, close_delay: 50, divisor: 0
        closing_wait: 3000, closing_wait2: infinte
        Flags: spd_normal skip_test session_lockout

    Note:   If the setserial -ag command does not work, make sure you have read and write permission for the port.

For all supported UNIX platforms, use the stty command to display or configure serial port information. For example, to display serial port properties for ttyS0, enter:

stty -a < /dev/ttyS0

To configure the baud rate to 4800 bits per second, enter:

stty speed 4800 < /dev/ttyS0 > /dev/ttyS0

Using Virtual USB Serial Ports

If you have devices that present themselves as serial ports on your operating system, you can use them as virtual USB serial ports in MATLAB. An example of such devices would be a USB Serial Dongle. For Bluetooth® devices, use the Bluetooth support in the Instrument Control Toolbox™. See Bluetooth Interface Overview for more information.

MATLAB can communicate with these devices as long as the serial drivers provided by the device vendor are able to emulate the native hardware. Certain software, like HyperTerminal, does not require the device driver to fully implement and support the native hardware.

Selected Bibliography

[1] TIA/EIA-232-F, Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange.

[2] Jan Axelson, Serial Port Complete, Lakeview Research, Madison, WI, 1998.

[3] Instrument Communication Handbook, IOTech, Inc., Cleveland, OH, 1991.

[4] TDS 200-Series Two Channel Digital Oscilloscope Programmer Manual, Tektronix, Inc., Wilsonville, OR.

[5] Courier High Speed Modems User's Manual, U.S. Robotics, Inc., Skokie, IL, 1994.

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