Input register is a function diagram element that allows the user to get information about controller's state.
All the controller resources that can be accessed from the function diagram – system registers, IO subsystem and additional drivers included in the system software – are located within the protected address space of the controller's internal memory.
By using the "Input register" function block the user is able to extract and use controller status data and information received by the controller from the outside world. For example: information about the electric potential at any contact of the controller; the contents of a data packet received over the CAN bus.
Refer to documentation for your specific controller model for more information.
The "Input register" element can be connected to a network, directly to a function block input or directly to a write register, thereby setting them to a value that changes synchronously with value changes in the corresponding register of the controller.