RS232 testing. How to test a serial port?



You have just connected a serial device to your machine’s COM port. Data acquisition software has been installed but the expected data does not appear. This is not acceptable and you think the issue is related to the RS232 communication. You decide you might need to do some serial port testing. The question is how to test a serial port?

Download the Serial Port Tester application. It provides RS232 tester software that allows you to gain insight into your serial communication to identify and fix errors.

When you are having difficulty obtaining data when using COM Port Tester, try these steps:

  1. Set the COM port details
  2. Use the Serial Port Tester
  3. Check the Cabling
  4. Set Handshaking or Flow Control
  5. Return to the Serial Port Tester

Set the COM port details


Collect all the information that you can regarding the serial device you will be connecting to your machine. You will be entering this data into the serial port tester software prior to its use. The information you need to provide includes the number of serial ports to which your device is connected, its parity, the number of data bits per byte, and the baud rate. If you don’t know the number of stop bits, you should set it to 2. This may slow down messages slightly but will cause no further harm. Consult documentation regarding making measurements through a serial port for detailed information regarding each setting. Using the COM Port Status window in Serial Port Tester you can query the status of lines going to and from the computer.

COM Port status bar

Serial port control information is presented as a string of 4 or 5 items, separated by a dash. In the descriptions below, “n” indicates a numeric item and “c” designates a character item.

  • nBaudRate: Defines the serial port baud rate. Possible values are: 110, 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 56000, 57600, 115200, 128000 and 256000. A default value of 110 is used if no other value is provided.
  • nDataLength: Defines a packet’s data length. Possible values are 5, 6, 7 and 8. The default is 7 bits.
  • cParity: Parity can be defined with one of these values: (E)ven, (O)dd, (M)ark, (S)pace, or (N)one. (N)one is the default.
  • nStopBits: Defines the number of stop bits. Possible values are: “1”, “1,5” and “2”. Default value is “1”.
  • cHandflow: Defines the flow control value. Possible values for this parameter are Hardware (P), Xon/Xoff (X) and None. The default setting is None.

Use the Serial Port Tester


Once the settings are in order you can begin a new testing session by following these steps.

1. Launch Serial Port Tester

2. Choose “Session >>> New session” from the main menu. You can also use the keyboard shortcut “CTRL+N” or click “New” on the main toolbar.

New session

3. The “New monitoring session” window will be displayed.

New monitoring session
  • Line view displays detailed information about the requests that are sent on a specific serial line.
  • Table view presents the recorded IRPs in table format.
  • Terminal view presents the received data in a text console of ASCII characters.
  • Modbus view displays received and sent Modbus data (RTU and ASCII).
  • Dump view displays all sent and received data transmitted through a serial line.

The checkboxes designated as “Start monitoring now” and “Start in new window” control how the new monitoring session is initiated.

Capture options: select the options you wish to monitor from among – Create/Close, Read/Write, and Device Control. When you are finished setting the options, activate the new session by clicking the “Start monitoring” button.

4. The new monitoring window is displayed containing the selected visualizers.

The process of monitoring

5. Save a session by selecting “Session >>> Save session/Save session As” from the main menu. You can also click “Save” on the main toolbar or use the CTRL+S keyboard shortcut.

Save testing session

6. When prompted, select a file name. Use this file to reload the session at a later time to continue the working with it.

Check the Cabling


When using a DTE instrument you may need to employ a null-modem adapter to test COM ports. You can also try to rewire the cable connector if you are feeling adventurous.

  1. The first wires to connect are the ground, RXD, and TXD.

  2. Using the device’s documentation, attempt to identify the signal wire that carries output data and connect this with the computer’s RXD.

  3. The computer’s TXD needs to be connected to the signal that carries input data to the device. Be careful not to rely solely on the signals’ labels. The same signal can be an input or an output based on the type of device being used, for example, a computer or a modem.


It can be challenging to determine which type of device you are handling. Documentation often skips over this information, but you may be able to deduce it from some of the device’s other signals. Modem type devices will have DSR as an output with a counter using DTR as its output. Knowing a single signal’s direction allows you to figure out the rest of them. Some manufacturers reverse the labeling of data signals for modem types of equipment, so proceed carefully.

With correctly named signals you have:

  • a computer to computer link which connects TXD to RXD and RXD to TXD.
  • a computer to modem link connecting TXD to TXD and RXD to RXD.

Crossing connections in this way is known as a null-modem arrangement and you may need to buy an adapter to replicate this connection to conduct your COM port test.

Set Handshaking or Flow Control


Set Hardware Handshaking


Once you have correctly wired the signals you should be able to conduct your serial port test. If messages are still not getting through, tying the handshake lines might do the trick.

There are two reasons for using a handshake arrangement.

  • A computer that is not ready to receive data can stop a device from sending any.
  • A device can prevent data being sent from a computer if it is not ready to receive it.

Just because your device has the inputs and outputs which make it capable of handshaking does not mean that you have to implement it. Many manufacturers just put the signals on the plug since they are available from the device’s processor. Your best bet regarding handshaking is to begin by connecting possible handshake lines with fixed voltages. In this way, they won’t impact the device’s operation. You will find that many devices already have resistors tied to handshake lines. This enables you to not connect them if you don't need handshaking.

If no handshaking is implemented when you start to test a serial port, what indications will you get that suggest it may be required?

  • One indication that handshaking is needed is if the input buffers overflow causing the computer to miss part of the sent message.
  • A device may also miss a portion of messages sent by the computer which will negatively impact its operation.

If you need to use handshaking with the serial port test software, be advised that Serial Port Tester employs DTR / CTS handshaking. In this configuration, the computer will use its DTR output as a sign that it is ready to receive data. The CTS input is available to be controlled by a device to limit the computer’s ability to transmit data. The status of the CTS input on the computer is only relevant when hardware handshaking is enabled. With no hardware handshaking, this input is ignored, but the DTR output is kept at a high level so it can be used to tie unused inputs to your device.

In some cases you will want the DTR to be low and the RTS to be high. To accomplish this, the handshaking needs to be removed.

  • Connect pins 8 and 7 (i.e. CTS drives RTS).
  • Connect pins 1, 4, and 6. This should maintain the DTR line in the correct state, by connecting it to DCD and DSR.

This modification is usually made at the device side instead of the computer’s end of the cable. Make sure that data transmission and reception is still occurring on the cable.

Set Software Handshaking


Xon \ Xoff Handshaking is a software protocol that can be used to control data flow. Here’s how it works. When a computer is sending data to a device, the device sends a single Xoff character when it cannot accept any more data at the moment. This stops transmission, which will commence when an Xon character is received by the computer, signaling to restart the communication. Either the device or computer can send the Xoff and Xon character. You can select this type of handshaking in COM Port Tester if your device requires it.

Return to the Serial Port Tester


After you have resolved any issues by going through the steps outlined above, go back to step one and check the COM port lines status.

COM port lines status

Below the main table, the states of the serial control lines are displayed.

  • RTS : Request to Send line indicator
  • CTS : Clear to Send line indicator
  • DSR : Data Send Ready line indicator
  • DCD : Data Carrier Detect line indicator
  • DTR : Data Terminal Ready line indicator
  • RI : Ring line indicator

The color of the circles indicates the state of a particular line with green indicating a high level, red standing for a low level, and gray for an undetermined state.

Serial communication issues can be hard to diagnose and correct. There are several strategies that you can try, and we have discussed some of the more common methods. RS232 test software can be an invaluable tool in fixing your serial communication issues. Serial Port Tester from Eltima Software is an excellent solution. It is a flexible and versatile tool that provides the user with an advanced set of features. If you test or develop serial devices and applications, this tool should be in your software toolbox.

small logo Serial Port Monitor
#1 at Serial Port Software
Serial Port Monitor
Log and analyze serial port activity
4.8 rank based on 41+ users
Get a download link for your desktop
Submit your email address to get a link for quick download on your desktop and get started!
or