delphi多线程出现错误：“无法在此会话中启动更多事务”

如题，线程定义如下：unit Thread_Read;interfaceuses
  Classes ,SPComm,Forms,MP_P_BaseForm,MP_P_DMMain,Controls,SysUtils,Buttons,ComCtrls;type
  Thread_Rd = class(TThread)
  private
    { Private declarations }
  protected
    procedure Execute; override;
  public
    Spcomm_RW:TComm;
    constructor Create(str_ComName1: string; spcomm_RW1: TComm);
    procedure GetOutString;
    procedure ExcuteGetOutStr();
    procedure FreeControlByIndex();
    procedure ExcuteFreeControlByIndex();
    procedure SetInputString();
    procedure ExcuteSetInputStr();
    procedure ExcuteCOM1;
        procedure ExcuteCOM3;
  end;implementation
{ Thread_Read }
uses
  MP_62056_RW;constructor Thread_Rd.Create(str_ComName1: string; spcomm_RW1: TComm);
begin
   Str_ComName:=str_ComName1;
  Spcomm_RW:= spcomm_RW1;
  inherited Create(False);
end;procedure Thread_Rd.Execute;
  { Place thread code here }
begin
  if Str_ComName='COM1' then
     Frm_62056_RW.ReadOperateCOM1  //为主窗口的过程
  else if Str_ComName='COM2' then
      Frm_62056_RW.ReadOperateCOM2
  else if  Str_ComName='COM3'  then
      Frm_62056_RW.ReadOperateCOM3
end;在主线程开启线程：
procedure TFrm_62056_RW.bbtn_ReadClick(Sender: TObject);
begin
     Thread_ReadCOM1:= Thread_Rd.Create('COM1',spcomm_RW1);
     Thread_ReadCOM2:= Thread_Rd.Create('COM2',spcomm_RW2);
end;
procedure TFrm_62056_RW.ReadOperateCOM1;
var
    Str_Error:string;
begin
    try
        if ReadDataProcess(Gl_Str_MTypeNo,Gl_Str_MeterNo,'COM1',Str_Error,Memo_Debug1,spcomm_RW)<0 then//此处出现如题错误 
       begin
            ShowWarningWithMsg(Str_Error);
        end else
        begin
            ShowInformation(sb_Object.Panels[con_PnlNo_ObjectCount].Text+','+
                            sb_Object.Panels[con_PnlNo_SucceedCount].Text+','+
                            sb_Object.Panels[con_PnlNo_FailedCount].Text);
        end;
    finally
        if bbtn_Read.CanFocus then bbtn_Read.SetFocus;
    end;
end;procedure TFrm_62056_RW.ReadOperateCOM2;
var
    Str_Error:string;
begin
    try
        if ReadDataProcess(Gl_Str_MTypeNo,Gl_Str_MeterNo,'COM2',Str_Error,memo_Debug2,spcomm_RW2)<0 then
        begin
            ShowWarningWithMsg(Str_Error);
        end else
        begin
            ShowInformation(sb_Object.Panels[con_PnlNo_ObjectCount].Text+','+
                            sb_Object.Panels[con_PnlNo_SucceedCount].Text+','+
                            sb_Object.Panels[con_PnlNo_FailedCount].Text);
        end;
    finally
        Class_GVar.Str_ExecLockSys:=con_ExecLockSys_Yes;
        if bbtn_Read.CanFocus then bbtn_Read.SetFocus;
    end;
end;求关注！！！先谢谢各位了多线程Delphi