I am trying to send a message from a MDI child window to the main window -- How do I do that? Is there a way with out using the API?

the reason is I have several MDI-Childs that represent an instrutment on a serial buss. All instrument must communicate via the same comport to my application (of several MDI-Childs windows). But I need to synronize the data going through the serial port.
(I'm somewhat new to Vb but worked in Visual C++ for a while. and I know too do it C++ but I must use vb)

Please help!