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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 = "MSCOMMLib.MSComm.1" $obj = ObjCreate($sAxName) With $obj .CommPort = 31 .Settings = "9600,N,8,1" .PortOpen = True .NullDiscard = False .NullDiscard = True .HandShaking = 0 .InputMode = 1 .DTREnable = True .EOFEnable = False .RTSEnable = True EndWith For $iCnt = 0 To 15 With $obj .NullDiscard = False $sMsg = "*A" & Chr(0) & Chr(Int($iCnt)) .Output = $sMsg If $iCnt < 3 then MsgBox(0, Default, $iCnt) EndIf EndWith Sleep(200) Next You might expect .NullDiscard to be relevant. The VB works with this true or false, and I've tried both in AutoIt. It's possible that I have some other property different, but I've looked through them all. I wonder if AutoIt is passing the whole string to the control, or stopping at the zero. I can't think how to prove that either way. Chr(Int($iCnt)) works correctly for values > 2, but for 1 it doesn't do the same as Chr(1) - what am I missing here?