fROTransaction := TpFIBTransaction.Create(Self);
  fROTransaction.TRParams.Add('read');
  fROTransaction.TRParams.Add('nowait');
  fROTransaction.TRParams.Add('rec_version');
  fROTransaction.TRParams.Add('read_committed');
...

  fROTransaction.StartTransaction;

procedure TpFIBTransaction.StartTransaction; //override;
begin
 if InTransaction then
  Exit;
 with TRParams do
  if  FTPBMode in  [tpbReadCommitted, tpbRepeatableRead] then
  begin
   Clear;
   Add('write');
   Add('isc_tpb_nowait');
   case FTPBMode of
     tpbReadCommitted :
     begin
      Add('read_committed');
      Add('rec_version');
     end;
     tpbRepeatableRead:
      Add('concurrency');
   end
  end;
...