bit hanging

we r facing a problem of bit latching which is connected to s7-400 through wincc like start and stop button,can any one say why is this happening??????

Without looking at the code, no, no one can tell you why it is happening. Do you have more info?

One thing I can tell you is that software does exactly what it's programmed to do. You will have to start at this bit and work backwards and just figure it out.