How to read serial port data?

Olga Weis

This article is intended to help you understand the basics of the hardware and software (protocol) standards for RS232. It will also show you how to use the COM port reader software to read data traveling through RS232 devices in real time.


  1. What does RS232 stand for?
  2. How to read data from an RS232 port?
  3. Main features of Serial Port Reader for Windows.
  4. How to capture serial port data on Windows with COM Port Reader?
  5. Understand RS232 Connection and Signals

What does RS232 stand for?

Serial ports, also known as RS232 ports, are an interface used for serial communication that transmit a single bit at a time. COM or communications ports are another name for serial ports.

The use of serial ports has declined with the development of USB and other high-speed solutions, but they still are used in some very important applications. Serial ports are instrumental in the operation of industrial automation systems and are often used to connect lab equipment and other scientific instruments to computers and networks.

It is imperative that you regularly monitor and analyze serial port activity when working with serial devices. You want to be able to see traffic flowing in both directions from your RS232/422/485 ports. This could be event notifications from apps, status messages or other information that enables you to troubleshoot the equipment if necessary and can serve as a guide for installing new devices.

How to read data from a serial port?

There is no specific operating system tool or function that allows you to read serial ports in Windows. But there is a solution, as software is available that can check, monitor and analyze RS232 port activity. It gives you a tool for Windows that can read COM ports.

Serial Port Monitor by Eltima Software

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Serial Port Monitor [Professional Edition]

Requirements: Windows (32-bit and 64-bit): XP/2003/2008/ Vista/7/8/10, Windows Server 2012. 9.16MB free space. Version 7.0.342. (15th Jan, 2018). 4.8 Rank based on 41+ users
Serial Port Reader is a powerful and user-friendly serial data recorder that gives you the capacity to begin reading any serial port in a system at any time, even if the port is already in use by another application.
Serial Port Monitor

COM Port Reader is a professional-grade software tool that should be your first choice when seeking an answer to the question of how to read data from RS232 ports.The tool allows you to send commands or other information to COM-based devices or RS232 applications in a variety of formats (string, binary, octal, decimal, hexadecimal, mixed). The returned responses can be monitored and saved in a single log file that employs the first-in-first-out method. Serial Port Reader also lets you redirect serial port output a file or the clipboard, so it is available for further analysis at any time.

Main features of Serial Port Reader for Windows:

• Reading COM port activity

This software utility allows you to read RS-232 data from a designated port and monitor it even if another application had already opened it. Captured serial data can be displayed in various formats, and the opportunity of real-time monitoring is a great feature for problem resolution.

The received data can be saved to a file of your choice or copied to the clipboard. The tool displays and files input/output control codes (IOCTLs) along with their complete parameters. Sessions can be saved by Com Port Reader and can be reloaded if required.

• Working with multiple ports in one session

Multiple serial ports can be read simultaneously by this software tool. This feature is very useful when comparing data collected from different COM ports that are interacting with the same application within monitoring session. In this case, all data is received and stored in a single log file on a first-in-first-out basis.

• Multiple views for sniffed data

Serial Port Reader allows you to choose the way that collected data is displayed on your computer. Four different view are available: table, line, dump, or terminal. You have the option of monitoring all view modes at the same time.

• Emulating serial communication

An option in terminal mode allows simulated data transmission from a serial application to a monitored COM port. Various data formats, such as string, binary, octal, decimal, hexadecimal, or mixed, can be used to test the COM port or its attached device’s reactions.

• Capturing Modbus data

COM Port Reader’s powerful filters enable you to read serial data transmitted over Modbus RTU and Modbus ASCII. The utility is fully compatible with these protocols, as well as those employed in RS-232, RS-485, and RS-422 interfaces

• Repetitive data exchange

Sending the same command from a serial app to a monitored serial port multiple times can give a clearer picture of the port’s behavior. This serial port utility simplifies that task with a playback feature that can display differences between sessions automatically.

Steps to capture serial port data on Windows with COM Port Reader

This how to read serial port data and collect it using COM Port Reader. First you need to download the tool, install and launch it. Then you need to start a monitoring session like this:

      1. In Serial Port Reader go to the Main menu, choose Session -> New session. Alternately, you can click on the ‘New’ icon on the main toolbar or press ‘Ctrl + N’. This invokes the ‘New monitoring session’ screen.

Serial Port Reader

      2. Select which view modes that you want displayed during your monitoring session:

Serial Port Reader view modes
  • Table view – recorded IRPs are displayed in the form of table
  • Line view – requests passed through a particular serial line are displayed along with details
  • Dump view – shows data passed through the serial line
  • Terminal view – all received data is displayed in ASCII characters on a text console.
  • Modbus view displays received and sent Modbus data (RTU and ASCII).

A ‘Select all’ and ‘Select none’ button are there to simplify your selections.

      3. Choose, whether you would like to ‘Start monitoring now’ or ‘Start in new window’ for the immediate behavior of a new session.

Serial Port Reader new session

      4. In ‘Capture options’ you can specify the events you are interested in capturing – Create/Close, Read/Write, Device Control.

      5. Click ‘Start monitoring’ to activate the session. A new monitoring window will be displayed.

      6. To save the session, in the main menu select ‘Session -> Save session/ Save session As’. Alternatively you can click ‘Save’ icon on the main toolbar or press ‘Ctrl + S’.

Give your session a meaningful name so you can return to it for later analysis.

Understand RS232 Connection and Signals

• DTE and DCE

DTE stands for Data Terminal Equipment. An example of a DTE is a computer. DCE stands for Data Communication Equipment. A modem is an excellent example of a DCE.

A DTE normally comes with a Male Connector, while a DCE comes with a Female Connector. This is not always the case. Here is a simple test to confirm device type. Using a volt meter, measure Pin 3 and Pin 5 of a DB-9 Connector. DTE devices will indicate a voltage of -3V to -15V. DCE devices will have the voltage on Pin 2.

Note: The result for a DB-25 Connector is reversed (Please refer to DB-9 to DB-25 conversion table below).

DB-25 Connector

• DB-9 to DB-25 Conversion

DB-9 to DB-25 Conversion

• RS-232 Connections

Straight-through cables are used to connect a DTE (e.g. computer) to a DCE (e.g. modem), with all signals in one side connected to the corresponding signals in the other side in a corresponding one-to-one basis. When connecting two DTE devices directly with no modem in between, a crossover, or null-modem cable is used. This type of cable cross transmits and receives data signals between the two sides. There is no standard and many variations on how the other control signals are wired. Below is an example of one of them:

RS-232 Connections

• RS-232 Signals

RS-232 Signals

The graphic above illustrates a typical RS-232 logic waveform (Data format: 1 Start bit, 8 Data bits, No Parity, 1 Stop bit). Data transmission begins with a Start bit, followed by the data bits (LSB sent first and MSB sent last), and ends with a "Stop" bit.

The voltage of Logic "1" (Mark) is between -3VDC to -15VDC, while the Logic "0" (Space) is between +3VDC to +15VDC.

RS-232 connects the Ground of 2 different devices together, which is the so-called "Unbalanced" connection. Unbalanced connections have a distance limitation of 50 ft (approximately 15 meters) and are very susceptible to noise.

Serial Port Monitor

Requirements: Windows XP/2003/2008/Vista/7/8/10/Server 2012 , 9.16MB size
Version 7.0.342 (15th Jan, 2018) Release notes