I am happy to write that I have finally been able to communicate with the controller! So the issue really was with the Minicom and I should have written a small program way before and would have saved a lot of time i wasted in trying to communicate using a shortcut method.
I ran into some problems in writing program such as setting the read() call to canonical input instead of raw input method. But I really appreciate ROD deep from my heart because whatever he suggested was the deciding factor in solving this issue (such as the links to the files in understanding the serial communication).
I have written the program to communicate with the controller, sending it out the command in hex mode and reading in the data the controller sends out to the laptop. In my case I wasn't really concerned what controller replies, rather I was interested only in sending out a proper HDLC command that conveys to the controller what I want it to show on the message sign.

Wait! this post is not done yet! I might come back to understand more about serial communication maybe soon. Thanks a lot Rod for great help -