Just browsing this forum, I found this topic on RS232 and Visual Basic.
Here goes a small software I did to interfase a DSP through the serial port. Check the subroutines and you will find how to communicate, send and receive characters.
You can try this easily: Get a computer with two serial ports (almost all new computers have two). Use a crossed serial cable (Rxd and Txd pins crossed) or just use wire wrap and connect the pins manually. Use your VB interfase connected to one port, and Hyperterminal connected to the second. Whatever you send in both, you should receive in the other one.
Hope it helps,
Mariano