def get_voltage(self, channel):
        """ channel: 1=OP1, 2=OP2, AUX is not supported"""
        ret = self.ask("V%dO?" % channel)
        if ret[-1] != "V":
            print("ttiQl355tp.get_voltage() format error", ret)
            return None
        return float(ret[:-1])
channel: 1=OP1, 2=OP2, AUX is not supported