我用vacomm发送数据，为何中途会有“output buffer is full”提示，咋处理啊

unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls, StdCtrls, ComCtrls, SPComm, jpeg, Unit2, VaClasses,
  VaComm;type
  TForm1 = class(TForm)
    GroupBox1: TGroupBox;
    Edit1: TEdit;
    Button1: TButton;
    Panel1: TPanel;
    Panel2: TPanel;
    ProgressBar1: TProgressBar;
    Label1: TLabel;
    Button2: TButton;
    Button3: TButton;
    Edit2: TEdit;
    Label2: TLabel;
    SpComm: TComm;
    OpenDialog: TOpenDialog;
    Image1: TImage;
    Label3: TLabel;
    Edit3: TEdit;
    VaComm: TVaComm;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure VaCommTxEmpty(Sender: TObject);
  private
    FileHandle, // 文件名柄变量
    FileLength, //源文件大小变量
    num,        //源文件被分割后的文件数目
    iBytesRead : integer;
    buf1 : integer;    //文件缓冲区变量    { Private declarations }
    function wFileSize(AFileName:string):integer;
  public
    { Public declarations }
  end;const
  AFileSize=255;
var
  Form1: TForm1;implementation//uses Unit2;{$R *.dfm}{procedure BreakFile(FileName:string);
  function CopyDataToSmallFile(S:TFileStream;Index:integer):integer;
  var
    T:TFileStream;
  begin
    Result:=0;
    S.Position:=Index*AFileSize;
    if S.Position>=S.Size then EXIT;
    T:=TFileStream.Create(Format('%s_%d',[FileName,Index]),fmCreate);
    try
      Result:=T.CopyFrom(S,AFileSize);
    finally
      T.Free;
    end;
  end;
var
  S:TFileStream;
  Index:integer;
begin
  S:=TFileStream.Create(FileName,fmOpenRead or fmShareDenyNone);
  try
    Index:=0;
    while CopyDataToSmallFile(S,Index)=AFileSize do
      Inc(Index);
  finally
    S.Free;
  end;
end;
}function TForm1.wFileSize(AFileName:string):integer;
var
F: file of Byte;
begin
  try
    AssignFile(F,AFileName);
    Reset(F);
    Result:=FileSize(F);
    CloseFile(F); //
    FileLength:=Result;
  except
    Result :=0;
  end;
end;procedure TForm1.Button1Click(Sender: TObject);
begin
  if OpenDialog.Execute then
  begin
    edit1.Text:=OpenDialog.FileName;
    Image1.Picture.LoadFromFile(OpenDialog.FileName);
    edit2.Text:=inttostr(wFileSize(OpenDialog.FileName));
  end;
end;procedure TForm1.Button2Click(Sender: TObject);
begin
  Form2.ShowModal;
end;procedure TForm1.Button3Click(Sender: TObject);
var Buffer:array of PChar;
    i,temp:integer;
    pszBuffer: pChar;
    Comming:boolean;
begin
  buf1:= AFileSize;
  num := Round(FileLength/AFileSize+1);
  SetLength(Buffer,num+2);
  //ShowMessage(inttostr(num));
  temp:=buf1;
  ProgressBar1.Max:=num;//设置进度条状态
  ProgressBar1.Position:=0;
  Comming:=true;
  //SpComm.StartComm;
  for i:=1 to num do
  begin
    pszBuffer := PChar(AllocMem(buf1));
    try
      if i=num then
      begin
        pszBuffer := PChar(AllocMem(FileLength-buf1*(num-1)));
        temp:=FileLength-buf1*(num-1);
      end;
      FileHandle:=FileOpen(edit1.Text,fmOpenRead);
      FileSeek(FileHandle,buf1*(i-1),0);
      iBytesRead := FileRead(FileHandle, pszBuffer^, temp);      FileClose(FileHandle);
      Comming:=VaComm.WriteChar(Char(pszBuffer));
      //spcomm.ByteSize:=0;
      if not Comming then
      //{Pif not spcomm.WriteCommData(pszBuffer,temp) then
      begin
        ShowMessage('发送错误，终止发送');
        //ProgressBar1.Max:=1;        ProgressBar1.Position:=0;
        break;
      end;
      ProgressBar1.Position:=i;
      sleep(5);
    finally
      FreeMem(pszBuffer);
    end;
  end;
  //ShowMessage('ok');
end;procedure TForm1.FormCreate(Sender: TObject);
begin
  VaComm.Open;
end;procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  VaComm.Close;
end;//我不太明白如何拍段缓存区是否空？如何把新的数据放进缓存里？
procedure TForm1.VaCommTxEmpty(Sender: TObject);
begin
  if VaComm.WriteBufFree>=1 then
     ;
end;end.