Вот мой не рабочий пример:

procedure TForm1.Button1Click(Sender: TObject);
var s:string;
begin
Comm1.DeviceName:='Com1';
Comm1.Databits:=TDatabits(da8);
Comm1.BaudRate:=TBaudrate(br19200);
Comm1.FlowControl:=TFlowControl(fcNone);
Comm1.Parity:=TParity(paNone);
Comm1.Stopbits:=TStopbits(sb10);
Comm1.Open;
Comm1.SetDTRState(true);
s:='ATE0'+#13;
Comm1.Write(s[1],length(s));
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
Comm1.Close;
end;