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

Question:

Are you looking for a way to create a FIFO queue and storage network using only basic logical instructions (and, or, not, xor)? I am currently working with a legacy PLC that lacks an expanded instruction set, and upgrading is not feasible due to cost and time constraints. For instance, imagine a scenario where boxes labeled 'a' and 'b' are moving along a conveyor belt and need to be sorted into separate containers based on their labels. The boxes are identified at point 'x' 50 feet before reaching the separator, where there is a brief transition time between containers. The boxes arrive in a random order. Your guidance on this matter would be greatly appreciated. Thank you.

Top Replies

Hey jendrus, could you please provide us with the brand and model number of the PLC you are using? Your input would greatly assist us. Thank you, James.

If you're unsure about its utility, the Genisys non-vital logic emulator could be just what you need. Take a look at the information provided in the programming manual excerpt below.

Is it possible to determine the feasibility of this task?

While this may not be what you had in mind, it might be beneficial to consider an upgrade if feasible.

jendrus asked if it is feasible to implement indirect addressing, pointers, array data types, etc on a particular device. Without these capabilities, it may be technically possible but could end up being more time-consuming compared to just upgrading the device. Can anyone provide insight on this?

In this case, you could use a shift register logic approach to queue and sort the boxes. When a box is identified at point ‘x’, store that information in the first bit of a shift register, let's call it R. Every time the conveyor belt moves far enough to bring the next box into the identification point, shift the values in R one place to the right. The rightmost bit on the shift register would then control the separator for a given box since each bit position in R corresponds to a specific position on the conveyor belt in relation to the separator. While a basic logical approach might be slower than modern methods, it can still effectively accomplish the task given the constraint of using a legacy PLC.

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Frequently Asked Questions (FAQ)

FAQ: 1. How can I create a FIFO queue and storage network using basic logical instructions in ladder logic?

Answer: - Answer: You can achieve this by using basic logical instructions such as AND, OR, NOT, and XOR in ladder logic to implement the required sorting and storage mechanism for the boxes moving along the conveyor belt.

FAQ: 2. Can you provide an example of how to sort boxes labeled 'a' and 'b' into separate containers using the FIFO queue approach?

Answer: - Answer: Sure, you can design a logic sequence that identifies the labels of the boxes at point 'x' and routes them to the appropriate containers based on their labels while maintaining the order of arrival using the FIFO queue concept.

FAQ: 3. How do I handle the brief transition time between containers in the sorting process?

Answer: - Answer: You can incorporate timing elements in your ladder logic program to manage the brief transition time between containers effectively and ensure the boxes are sorted accurately while maintaining the desired order.

FAQ: 4. Is it possible to implement a FIFO queue and storage network without needing to upgrade to a PLC with an expanded instruction set?

Answer: - Answer: Yes, it is possible to design and implement a FIFO queue and storage network using basic logical instructions in ladder logic, even on a legacy PLC that lacks an expanded instruction set, as long as you carefully plan and structure your logic program.

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