Creating a FIFO queue and storage network with basic logical instructions in ladder logic

What is a PLC and how does it relate to legacy PLCs? Specifically, if you are utilizing an AB PLC2 series, it comes equipped with a built-in FIFO function that can be easily utilized for your needs.

• 03-08-2024
• GaryS

When searching "programming Genisys and Microlok Plus" online, you can discover a wealth of information dating back to 1991. During that time, working with these systems was much more challenging compared to today. The complexity of these systems is on par with PDP-11 systems, and possibly even more obscure. It may be time for an upgrade to modernize the systems. Consider using a shift register as a FIFO, but it's important to determine if the instruction set includes a JMP or similar function to halt the FIFO when not in use.

• 03-08-2024
• danatomega

Although this unit does not support arrays, pointers, or indirect addressing, it communicates with other units through serial links using a specific protocol. While it may be tempting to replace it with an Allen Bradley unit, the upgrade would be a substantial project, requiring the replacement of multiple units or finding a way for the Allen Bradley unit to communicate with the other units effectively.

• 03-08-2024
• jendrus

Take a glance at page 15 of the document to uncover the challenges faced in the past when implementing a hardware FIFO. While the translation to programmed logic is straightforward, coding for longer lengths can be quite daunting. This particular example focuses on a single bit, but to handle bytes, you'll need to work with eight bits in parallel. Considering the context of boxes a and b, it seems likely that a single bit may suffice.

• 03-08-2024
• jvdcande

How many FIFO file locations are you referring to? Essentially, you will act as the FIFO system. You must devise a method to track the FIFO position by using a list of bits, and ensure that FIFO file elements are properly shifted during loading and unloading transitions. In simple terms, during a FIFO load operation, you should examine the position bit group to identify the highest zero value position bit. Then, you should set the FIFO bit associated with that position to the FIFO input state and update the FIFO position bit for that location. If you try to load and the highest possible position bit is already set, the load operation should be aborted. For unloading, you will first need to set the FIFO output to the value of FIFO file bit 0. Subsequently, you should update file bit 0 with the status of file bit 1, file bit 1 with the status of file bit 2, and so on, until you reach the FIFO file bit whose location bit is set to zero. Lastly, the highest non-zero FIFO position bit should be reset. If you attempt a FIFO unload and find that position bit 0 is NOT set, the unload operation should be aborted. Does this sound like a fun challenge to you?Keith

• 03-08-2024
• kamenges

It is theoretically possible to add and remove bits from a buffer by manually shifting each memory bit. For instance, you can add a new bit using a code like the one below: To add a new bit to the "buffer": Code: C -> Add -> D -||---||---() D -> Add -> B -||---|/|-|C->DB->CA->BNew->A To remove a bit from the buffer, you would need to track the buffer's length and extract the data accordingly. You may require a parallel buffer with only one true bit to determine which element of the FIFO buffer to read from. This would involve writing read commands for all possible elements, with only one executing at a time, followed by adjusting the parallel buffer accordingly. In practical terms, managing a FIFO buffer deeper than 3 or 4 layers manually can become tedious. If you are communicating with other systems via serial communication, consider incorporating a modern PLC dedicated to operating the FIFO stack. This PLC could streamline the process and facilitate communication with the system.

• 03-08-2024
• mk42

mk42, I believe you are exhibiting a FILO approach, which aligns with the core concept. - Keith

• 03-08-2024
• kamenges

kamenges pointed out that the concept displayed by mk42 is similar to a FILO (First In, Last Out), although the underlying idea is correct. Keith agreed, commenting that he usually uses a stack buffer instead of a FIFO (First In, First Out) and will make the necessary adjustments.

• 03-08-2024
• mk42

It seems that the manual excerpt does not display the complete set of instructions. If you are working with word variables and have the ability to manipulate their individual bits using a SHIFT RIGHT operation, you can try the following approach: Create a variable called FIFO WORD and upon a certain step INPUT, shift the content in FIFO by 15 bits to the right, then perform a specific action based on the value of n depending on the queue length. If the queue exceeds 16, utilize another word variable FIFO2 and shift its content by 15 bits to the right and read from FIFO2.

• 03-08-2024
• ydtech

To effectively program a PLC, you will require development software, a simulator, and a compiler. An option to consider is utilizing an Automation Direct PLC for queuing tasks, using digital I/O to communicate the box's status to the MicrLok PLC at the diverting section of the system.

• 03-08-2024
• danatomega

In a discussion, mk42 suggests adding a new element to the buffer through a code snippet. To remove an element from the buffer, it is essential to keep track of its length and extract the data accordingly. It may be necessary to have a separate parallel buffer with only one active bit to indicate which element of the FIFO buffer to retrieve. Writing read commands for all potential elements and executing only one at a time is crucial. This process could become cumbersome when dealing with a FIFO buffer with more than 3 or 4 layers. My current usage involves a buffer with just 3 or 4 layers. However, I am unsure whether I need two buffers for two boxes or just one will suffice. Consideration is also given to utilizing a micrologix unit for managing the FIFO, even though it may require wiring and hardware adjustments.

• 03-08-2024
• jendrus

If upgrading is not an option, consider adding a new PLC to easily implement serial communications for your advanced logic needs. This will allow you to integrate the old, outdated system with modern technology and improve functionality.

• 03-08-2024
• rdrast

WARNING: NECRO THREAD DETECTED! Unfortunately, I missed out on this discussion. The solution is clear, yet laborious as mentioned by fellow users.

• 03-08-2024
• drbitboy