Thanks to liuzu2000 & Calvin. The problem I have now is that the PLC can read the data I sent from PC, but the PC can't read the data from PLC. Previously I have no problem with this. That was when I was using Win NT visual basic version 4. Is it PC operating system causing the communication problem? Because I'm using Win XP now. If XP causing the problem, can the problem be solved by using visual basic version 6 or later?