Stops SI manager thread
This safely stops the SI manager thread. When the SI manager thread is stopped in a manner outside its normal operation, there is a danger of the SI manager thread being completely stopped while it is waiting for a message from the SI. When this function is used, the thread is stopped after the SI manager is placed in a state in which it can be stopped. While the thread is stopped, the scheduler is set so that it will not send retrace messages.
Use this function when directly accessing SI devices from the user application.
To continue execution of the SI manager thread once it has been stopped, call the nuSiMgrRestart() function.