class ScalarDevice(RegisterBase):    
    def doRead(self):
        return self.val if hasattr(self, 'val') else 0.0 

    def doWrite(self, val):
        self.val = val        

    def test(self):
        self.setCache(1.0, None)


add_device(ScalarDevice("scan_start"), True)

scan_start.test()