Thanks for your quick reply. I do have a general idea of how to use SerialPort component for communicating with the deveices that are connected to serial port, but my problem is how to initiate the hardware to perform some function and then read the output from the hardware (loop tester). I have few lines of code written in VB6, which talk to the hardware with which fire detectors are connected in loop. This piece of code initiates the hardware to detect the numbers of devices in loop and then gives them all address (in hexadecimal). If you have some time from your busy schedule and are willing to view the code, I am ready to send you the code and output screen shots so that you have an idea of what i want to write exactly in C#. Let me know if you are interested and forward me your email address so that i can mail you the code . Thanks