Собственно пару функций для конвертера форматов (d4 & fpc), добавьте если нужно (для передачи по COM порту вероятно точно нужно)
Проверить можно тут http://www.binaryconvert.com/convert_double.html

Код:
Add(MainForm,2953706,28,161)
{
 Width=450
 Height=156
}
Add(InlineCode,10978967,154,119)
{
 WorkPoints=#13:doDoubleToHex|13:doHexToDouble|0:|
 EventPoints=#5:onHex|6:onReal|
 Code=#15:unit HiAsmUnit;|0:|9:interface|0:|21:uses kol,Share,Debug;|0:|4:type|28: THiAsmClass = class(TDebug)|10:   private|0:|9:   public|29:     onHex,onReal: THI_Event;|55:     procedure doHexToDouble(var dt: TData; idx: word);|55:     procedure doDoubleToHex(var dt: TData; idx: word);|5: end;|1: |49: function HexToDouble(const hex: string): Double;|47: function DoubleToHex(const d: Double): string;|0:|14:implementation|0:|36:procedure THiAsmClass.doHexToDouble;|5:begin|49:  _hi_OnEvent(onReal, HexToDouble(ToString(dt)));|4:end;|0:|36:procedure THiAsmClass.doDoubleToHex;|5:begin|46:  _hi_OnEvent(onHex, DoubleToHex(ToReal(dt)));|4:end;|0:|48:function HexToDouble(const hex: string): Double;|3:var|12:  d: Double;|45:  Overlay: array[1..2] of LongInt absolute d;|5:begin|29:  if LENGTH(hex) <> 16  then |49:    _Debug('Invalid hex string for HexToDouble');|72:  //  then raise EIEEEMath.Create('Invalid hex string for HexToDouble');|42:  Overlay[1] := cHex2Int(COPY(hex, 9, 8));|42:  Overlay[2] := cHex2Int(Copy(hex, 1, 8));|14:  Result := d;|4:end;|0:|46:function DoubleToHex(const d: Double): string;|3:var|45:  Overlay: array[1..2] of LongInt absolute d;|5:begin|73:  // Look at element 2 before element 1 because of "Little Endian" order.|60:  Result := Int2Hex(Overlay[2], 8) + Int2Hex(Overlay[1], 8);|5:end; |1: |4:end.|
 link(onHex,15426071:doText,[])
 link(onReal,5127712:doText,[(222,132)(222,174)])
}
Add(Edit,3844880,84,63)
{
 Left=15
 Top=20
 Text="1.7"
}
Add(Button,9185643,28,119)
{
 Left=80
 Top=20
 Width=85
 Caption="RealToHex"
 link(onClick,4444644:doData,[])
}
Add(Edit,15426071,245,119)
{
 Left=190
 Top=20
 Width=225
 Text=""
}
Add(DoData,4444644,84,119)
{
 link(onEventData,10978967:doDoubleToHex,[])
 link(Data,3844880:Text,[])
}
Add(Edit,9926444,84,161)
{
 Left=15
 Top=60
 Width=235
 Text="3FF0000000000000"
}
Add(Button,5509348,28,210)
{
 Left=265
 Top=60
 Width=85
 Caption="HexToReal"
 link(onClick,13471617:doData,[])
}
Add(Edit,5127712,245,168)
{
 Left=365
 Top=60
 Text=""
}
Add(DoData,13471617,84,210)
{
 link(onEventData,10978967:doHexToDouble,[(135,216)(135,132)])
 link(Data,9926444:Text,[])
}
Add(PointHint,5186153,35,14)
{
 Info=#65:для проверки -   http://www.binaryconvert.com/convert_double.html|
 Font=[MS Sans Serif,8,1,8388608,1]
 Width=456
}