Implementing FIFO and LIFO Data Sequencing in PLC Programming

〡 07 Jun, 2026
〡 by WUPAMBO

Data management serves as a cornerstone of modern industrial automation. Whether tracking materials on a conveyor or managing batch sequences in a process, engineers frequently rely on sequential logic. Two primary structures—First-In-First-Out (FIFO) and Last-In-First-Out (LIFO)—form the bedrock of this data handling. Mastering these blocks allows programmers to optimize complex machine operations efficiently.

Understanding the FIFO Sequence in PLCs

The FIFO (First-In-First-Out) principle operates on a simple premise: the first data element entered is the first one retrieved. In PLC programming, dedicated function blocks manage these queues. These blocks typically handle storage buffers that can scale significantly based on specific controller capabilities.

In my experience, FIFO blocks prove indispensable for tracking products through sequential assembly stages. The block requires three primary inputs: Reset, Storage, and Retrieval. A rising edge on the Storage input saves the current data word, while a rising edge on the Retrieval input pulls the oldest recorded value. Furthermore, status bits—"Empty" and "Full"—provide real-time feedback to the system, preventing data overflow.

Utilizing the LIFO Sequence in PLCs

Conversely, the LIFO (Last-In-First-Out) logic processes data in reverse order. Here, the most recent entry is the first to be retrieved. While FIFO is ideal for flow control, LIFO is often superior for stack management or specific "undo" operations in machine sequences.

From a technical standpoint, the LIFO block architecture mirrors that of a FIFO block. It utilizes the same Reset, Storage, and Retrieval inputs to manage its buffer. However, the internal pointer logic differs. Upon receiving a retrieval command, the controller identifies the most recently stored element. Therefore, developers must ensure input stability, as simultaneous pulses on storage and retrieval inputs can result in unpredictable block behavior.

Critical Considerations for Robust Programming

Effective implementation requires awareness of how controllers handle power transitions. For example, a cold restart usually clears the data registers entirely. In contrast, a warm restart might retain existing memory values, depending on the manufacturer's specific firmware design.

Always consult the documentation for your specific PLC or DCS hardware. Inconsistent handling of these buffers during power cycles can lead to system errors in production environments. Furthermore, I recommend implementing interlocks in your code to prevent simultaneous input signals. Maintaining clean, predictable logic ensures system stability and simplifies future troubleshooting for maintenance teams.

Practical Application Scenarios

FIFO Applications: Use this sequence for warehouse inventory tracking, conveyor belt product spacing, or sequence control in packaging lines where product order must remain static.
LIFO Applications: Implement this logic for specialized batch processes or nested cycle operations where the system must revert to the previous state after completing a sub-task.

About the Author

Lin Hao (林浩) is a veteran industrial automation engineer with over 15 years of global field experience. His career focuses on high-precision control system architecture, including PLC, DCS, and electrical protection schemes for critical industrial infrastructure. Known for his technical rigor, Lin specializes in optimizing factory automation workflows and providing strategic insights for complex, large-scale industrial projects.