10a shows the basic form a ladder program for such an application can take. When an item enters, it gives a pulse on input In 1. This increases the count by 1. Thus each item entering increases the accumulated count by 1. When an item leaves, it gives an input to In 2. This reduces the number by 1. Thus each item leaving reduces the accumulated count by 1. When the accumulated value reaches the preset value, the output Out 1 is switched on. Figure 10.10b shows how the preceding system might appear for a Siemens PLC and the associated program instruction list. CU is the count up input and CD the count down. R is the reset. The set accumulator value is loaded via F0.0, this being an internal relay.