How to test a serial port
RS232 testing guide

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You may need to test serial ports for a variety of reasons. One situation to consider is when you are not receiving an expected data flow. Perhaps you have data acquisition software that should be getting information from a device connected to your computer’s COM port. Unfortunately, there is no data being transmitted and you suspect you may have a communication issue. You want to identify the issue and will need to test the COM port. Now you just need to know how to test serial ports.

RS232 testing software enables you to test serial ports in Windows 10/8/7 to see exactly what is going on with your serial communication, This enables you to troubleshoot and diagnose communication issues. The first step is to download the RS232 tester software.

Here’s how to check if the RS232 port is working when you have problems with the data flow while using COM Port Tester.

  1. Set the serial port details
  2. Use the Serial Port Tester
  3. Serial cable testing
  4. Set Handshaking or Flow Control
  5. Return to the Serial Port Tester

Set the serial port details


Before you can use the serial port tester you need to provide details about the device you are attaching to your computer.

Gather all the device’s details such as the baud rate, the number of serial ports to which it is connected, data bits, and parity. You can use a value of 2 for the top bits if you don’t know how many are in use. This will not cause any harm except to possibly slightly slow down data transmission. You can query and monitor the status of current connections to your machine with Serial Port Tester’s COM Port Status window.

Note: Detailed information concerning each setting can be obtained by reviewing the device’s documentation.

serial port details to test COM port

Serial port control information is displayed as a collection of 4 or 5 items, each separated by a dash. In the fling list, a leading “n” indicates a numeric item and a “c” designates a character item.

nBaudRate:


Defines the serial port baud rate or transmission speed. Possible values are: 110, 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 56000, 57600, 115200, 128000 and 256000. If no value is set, a default of 110 is used.

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.

cHandflow:


Defines the flow control used in the connection. It can be set to Hardware (P), Xon/Xoff (X) and None. The default setting is None.

nDataLength:


Defines the length of a data packet. Possible values are 5, 6, 7 and 8. The default is 7 bits.

nStopBits:


Defines the number of stop bits. Possible values are: “1”, “1,5” and “2”. Default value is “1”.

How to check serial port communication with
Serial Port Tester


Follow these steps to initiate a new testing session after you have defined the settings for the connection.

  1. Launch the RS232 test software.
  2. Choose “Session >>> New session” from the main menu. Clicking “New” on the main toolbar or using “CTRL+N” are alternative methods of starting a session.
  3. start serial port test
  4. The “New monitoring session” window will be displayed.
  5. New testing session
  6. Choose how the session’s data will be displayed by selecting from among these visualizers:

    • Line view concentrates on a specific serial line and provides detailed information regarding any requests it receives.
    • Dump view displays all sent and received data transmitted through a serial line.
    • Terminal view uses an ASCII text console to display the data that is received through a line.
    • Modbus view displays received and sent Modbus data and support both the RTU and ASCII variation of the protocol.
    • Table view employs a table format to present the recorded IRPs.

You can control how a new monitoring session begins with the “Start monitoring now” and “Start in new window” checkboxes.

Select the capture options from these choices: Create/Close, Read/Write, and Device Control.

Click the “Start monitoring” button to start the new session after all the options have been set.

  1. A new monitoring window is opened using the visualizers previously selected.
  2. new testing window
  3. You save a session with the “Session >>> Save session/Save session As” option from the app’s main menu. Alternately you can use the CTRL+S keyboard shortcut or “Save” on the main toolbar.
Save serial port testing session

Specify a file name at the prompt so you can reload the session later to work with it further.

Serial cable testing


DTE equipment may require you to use a null-modem adapter to conduct a serial port test. Rewiring the cable connector is another option that more technically proficient users might attempt.

  1. First, you need to connect the ground, RXD, and TXD wires.
  2. Next, connect the wire that carries the device’s output data signal to the computer’s RXD. The documentation that comes with the device might guide you in finding this wire.
  3. The next step in performing a serial cable test is to connect the device’s input line to the computer’s TXD. Signal labels can be deceiving so be careful. Depending on the type of device, input or output wires can use the same signal.

Note: At times it can be difficult to identify the type of device in question. The documentation is often sketchy and you may need to use your deductive powers by looking at the device’s other signals. Devices such as modems have a DSR as an output and use the DTR as a counter.

Other manufacturers reverse this labeling, making it challenging to figure out the device’s signals.

With correctly named signals you end up with:

  • a Computer to Computer link connecting TXD to RXD and RXD to TXD;
  • a Computer to modem link that connects TXD to TXD and RXD to RXD.

The crossed connections described define a null-modem configuration. When deciding how to test a serial port, you may need to purchase an adapter to implement this connection.

Set Handshaking or Flow Control


Set Hardware Handshaking


You are ready to test RS232 ports once you have the signal wires connected correctly. If you are still not successful in receiving data, you might need to investigate the handshaking lines in order to test serial ports.

A handshaking arrangement is done for one of two reasons:


  • To allow a computer that is not ready to receive data to stop a device from transmitting any;
  • To enable a device to hold up communication from a computer if the device is not yet ready to receive the data.

Handshaking does not need to be implemented just because a device has the capability to do so. In many cases, manufacturers put the signals on the plug as they are easily available from the unit’s processor. If you choose to use handshaking, it is advisable to start with lines carrying fixed voltages. This minimizes any effects on the equipment’s operation. In cases where resistors are tied to handshake lines, you do not need to connect them.

There are some indicators that lead to the conclusion that handshaking should be implemented.


  • If an input buffer overflow causes the computer to miss part of a message, handshaking may solve the issue.
  • Similarly, a device can miss part of a message which will impact its ability to function correctly.

If you choose to implement handshaking with your RS232 tester, you should know that Serial Port Tester supports DTR / CTS handshaking. Using the tool, the computer employs its DTR output to indicate it is available to receive data. Devices can take advantage of the CTS input to limit the data flow from the computer.

The only time that the computer’s CTS input is important is when you are performing hardware handshaking. If you are not using hardware handshaking the CTS is ignored, but a high level is maintained on the DTR so you can use it to tie unused inputs to the device.

Removing handshaking will enable you to keep the RTS high and the DTR low. Here’s how to do it:


  • 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 change should be performed on the device’s end of the cable. After making the modification, ensure that data is still flowing through the cable.

Set Software Handshaking


A software protocol that enables Xon \ Xoff handshaking can also be used to control the flow of data between a computer and a connected device.

The protocol works by a device sending an Xoff character when it cannot receive any more data. Transmission is halted until an Xon character is sent and received by the computer which informs it to restart the communication. Xon and Xoff characters can be sent by the device or computer.

Software handshaking is supported by COM Port Tester if it is required by the device you are using.

Return to the Serial Port Tester


Once serial communication issues have been solved by the previously outlined steps, you can return to step one and check the status of your serial port lines.

Serial Port Tester

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

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

The RS232 port test is facilitated by a graphical display of the status of the lines. A green circle indicates a high level with red representing a low level and gray pointing to a line with an undetermined state.

It can be hard to pinpoint the cause of serial communication problems. Some of the more common methods of diagnosing the issue have been discussed in the article.

Using a quality software tool can help you identify and address issues quickly. Eltima Software’s Serial Port Tester offers an excellent solution that provides users with an advanced feature set for diagnosing serial communication problems. Its versatility and flexibility make this a valuable addition to the software tools available to individuals working with serial applications and devices.

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