The rung above has many instructions let’s examine them in detail. The FFU will remove an element at the end of the array into a specified tag. The FFL will transfer an element into the array while shifting everything else by a specified step size. The FIFO relies on two critical instruction: FFL and FFU. This is achieved through the first rung in our routine which can be seen below. Since our logic will require a time-stamp, we need to leverage the internal clock of the PLC system. FIFO Logic in RSLogix 5000 PLC Programming A traditional system will give you the number of cases from 6AM to 7AM and sometimes a rolling number starting from 7AM. The nuance here is that as you walk up to the production line at 7:43AM, you should be able to see how many cases were made from 6:43AM to 7:43AM. At first glance, it may seem that such systems already exist, but from our experience, most will report the quantity produced at every hour. In other words, the logic can be used to report a rolling average of the number of products made in the last hour. Today, we’re looking at an example which involves a FIFO Implementation and results in a way to store data based on short periods in order to compute a rolling average of such data. By practicing implementing such structure, you will build your knowledge of PLCs, the software package of your choice and you’ll become a more proficient programmer.

Within a short period of time, you may be asked to implement a simple logical structure such as a Motor Starter, a stack light system or a basic FIFO set of rungs.

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