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How To Troubleshoot COM Port Or RS-232 Controller

COM port, also known as RS-232 interface or serial interface, was once regarded as must-have interface port on every IBM-compatible PC as well as most server class computer system.

However, this low performance data port is almost obsolete from most modern computers and replaced by its successors, i.e. FireWire and USB port.
However, there are still some good old application and hardware (mostly are proprietary products) rely on RS-232 interface to function.

Thus, PCI-COM card or USB to Serial port are still available for sale in computer peripheral shops, to compensate the missing of built-in serial port controllers on latest motherboards.

However, as there not many users using COM port in the recent years, it’s a bit difficult to handle or conduct preliminary troubleshooting on serial interface related problem.

Not only NKOTB, old man like me as well “black-out” this morning, when a serial port receipt printer fail to print out single piece of paper! :-(

But, thanks god that I’ve a partner who know a bit of this. :-)

If you want to prove the first COM port is working fine, attach a known good dial-up modem to that COM port. Then, open up Windows Command Prompt window and enter this simple echo command (compatible with MS-DOS Command Prompt and maybe applicable to UNIX or Linux command prompt as well)

echo ata > com1:

The echo ata command will signal the good modem to raise squealing sound, which can be stopped by executing another echo ath command as this

echo ath > com1:

Other than dial-up modem, you can also rely on a RS-232 loopback connector with LED wired to the RS-232 signal pins. If the COM controller in question is working, then the LED will light up, otherwise nothing will happen.

Anyway, both dial up modem and loopback connector can hardly be seen nowadays, as many have been using broadband modem.

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  1. dan 03-07-08@01:24

    what pins are the loop back pins

  2. Walker 03-07-08@10:36

    Hi Dan, please refer to this for reference.

  3. Mahmood 29-12-09@01:48

    Dear sIrs, Good Day
    I have a trouble of ( Trasnmission Error) when try to pack up the Parameters from PC to CNC machine Via WINDNC.
    Please let me know how to tset RS232 Ports, Cable and to solve the error.
    Best Regards

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