I've tried to send 4 byte message via MSComm control VB and in AutoIt. Message is "*A<msb><lsb>". This code works in VB with any value 0 or 1 in msb, and 0 to 255 in lsb.
.Output = "*A" & Chr(0) & Chr(ix)
In AutoIt this works, but not if either byte is zero, and not if lsb is 1.
$sAxName...