I am new to the formal idea of UML diagrams, but I have drawn and read system type diagrams for years. I wish to set a very simple exercise for the analysis stage of a project. I am considering the operation of a checkout till with a built-in scanner and weighing device. I can capture the interaction of the checkout operator and the customer with the till with a Use Case diagram. However, what is the optimum type of diagram to show the high-level internal operation of the till such as “scan and interpret barcode”, “look up code” and output item name and price on display, “read payment card” etc.?