WPF MVVM accepts input from weight scale

I am working on an application that originally was done using the code behind for the xaml pages. I am converting it to use the MVVM pattern.  I have code that will accept the input from a usb scale populate the text box using DispatcherTimer  to poll the scale when I used the code behind format. But where should I put this code to populate the text box from the weight scale in the MVVM format? 

I am starting to get the hang of the MVVM pattern but still have a way to go, I appreciate all the help I have been getting.

Thanks you