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MQTT - Message Queue in TIA

Created by: libsch79 at: 2/9/2025 12:18 PM (7 Replies)

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2/9/2025 12:18 PM	Rate (0)
libsch79 Regular Member Joined: 4/28/2015 Last visit: 2/26/2025 Posts: 57 Rating: (1)	Hello, I have set up my PLC as an MQTT client using the Siemens library, and the connection to the broker is established. Now, I want to send the states of variables to the broker using different topics. The messages should be sent when a value changes. I use a positive or negative edge to pass the topic and payload to the MQTT function block. After that, I trigger the send signal again with the edge. Now my question: If multiple variables change within one program cycle, I need a queue. The messages should be sent one after another until the queue is empty. How can I implement this with the S7-1500? Best regards, Daniel

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2/10/2025 11:01 AM	Rate (1)
jacek d Platinum Expert Joined: 9/23/2005 Last visit: 1/14/2026 Posts: 5155 Rating: (772)	LGF - there you can find LGF_FIFO - an implementation of a queue.
	Regards, Jacek What is confusing you is the fact that you never see a number but always its digit representation. So you end up thinking that the number is the representation. - Andrej Bauer Any claims against me - based on whatever legal reason - resulting from the use of any code and/or ideas presented on this forum shall be excluded. #StayWithUkraine
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This contribution was helpful to 3 thankful Users libsch79 0_Einstein Edu2020

2/10/2025 6:30 PM	Rate (0)
libsch79 Regular Member Joined: 4/28/2015 Last visit: 2/26/2025 Posts: 57 Rating: (1)	Thank you. I will try it with that.

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2/14/2025 5:20 PM	Rate (0)
libsch79 Regular Member Joined: 4/28/2015 Last visit: 2/26/2025 Posts: 57 Rating: (1)	The FIFO function block does not solve my problem either, if I understand its functionality correctly. The enqueue input of the block is also processed only once per program cycle. This means I can only store one item per cycle in the buffer. Attachment Screenshot 2025-02-14 181621.png (150 Downloads)

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2/14/2025 5:35 PM	Rate (0)
0_Einstein Grandmaster-Expert Joined: 7/7/2010 Last visit: 1/14/2026 Posts: 16369 Rating: (2522)	Here's where things are possibly going to blow your (and other people's) mind! Does your FIFO block currently successfully enqueue within one block call? If so, the enqueue input needs to drop low and high again which will require 2 PLC scans, OR, a call to the FIFO block with a FALSE input to both ENQUEUE and DEQUEUE inputs so it can start immediately looking for another enqueue. In the very next LAD network, add a bit of control logic monitoring the FIFO output from the current FIFO "enqueue" block. If the last request had no problem, perform another FIFO enqueue by sticking into another call to the FIFO block using different input item details but the same FIFO instance DB. So, to summarize: LAD # n: enqueue = true => Existing FIFO LAD # n+1: enqueue = false => Copy of Existing FIFO (same instance DB) LAD # n+2: enqueue = true + new item => Copy of Existing FIFO (same instance DB) LAD # n+1: enqueue = false => Copy of Existing FIFO (same instance DB) In this way, every PLC scan you can add TWO FIFO entries. However, how fast is your MQTT going to transmit? Wouldn't it be better to create a JSON blob with all the data to be sent in each MQTT transmission rather than a bunch of smaller ones? How are you managing your MQTT sending? Fill the queue and send; wait for send to complete while filling queue; send again once queue is "full" or send as soon as last send completes? I think you should only be sending on demand or when something changes, and only per some predefined schedule. libsch79 The FIFO function block does not solve my problem either, if I understand its functionality correctly. The enqueue input of the block is also processed only once per program cycle. This means I can only store one item per cycle in the buffer.
	science guy
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This contribution was helpful to 2 thankful Users jacek d libsch79

2/15/2025 10:50 AM	Rate (0)
libsch79 Regular Member Joined: 4/28/2015 Last visit: 2/26/2025 Posts: 57 Rating: (1)	Thanks for your response. This is a project with my students. I want to introduce them to the functionality of MQTT and OPC pub/sub. Currently, the PLC is acting as the OPC server, and I am using Node-RED as the client. We are polling the individual variables cyclically. However, it would be better if the PLC pushed the values only when they change, reducing unnecessary traffic on the network when no changes occur. I also have an issue with the writer group in OPC pub/sub, but that’s for another post. That’s why I tried using MQTT. My idea was to use a separate topic for each variable. This way, I can demonstrate to my students that data is only transmitted when a value changes. Additionally, I can show them one topic, and they can create the rest on their own. That’s why I am trying to transmit the values individually. The project is just a simple traffic light. For example, red and orange turn off, and green turns on—all within one cycle. Of course, I could design the switching so that only one change occurs per cycle. However, I find using a queue more reasonable since MQTT inherently works with queues. This would also allow me to demonstrate that concept. An alternative approach would be to write all variables into an array-of-byte and send the whole packet once per cycle if something changes. However, this would not be ideal for my planned lesson.

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2/17/2025 9:02 AM	Rate (0)
jacek d Platinum Expert Joined: 9/23/2005 Last visit: 1/14/2026 Posts: 5155 Rating: (772)	0_Einstein Here's where things are possibly going to blow your (and other people's) mind! Be warned. This technique can be used in different execution levels (i.e. in different tasks). Therefore data access concurrency can occur. Consistency of data must be assured. Otherwise not only mind will be blown but also whole subtile plan.
	Regards, Jacek What is confusing you is the fact that you never see a number but always its digit representation. So you end up thinking that the number is the representation. - Andrej Bauer Any claims against me - based on whatever legal reason - resulting from the use of any code and/or ideas presented on this forum shall be excluded. #StayWithUkraine
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2/17/2025 2:57 PM	Rate (0)
0_Einstein Grandmaster-Expert Joined: 7/7/2010 Last visit: 1/14/2026 Posts: 16369 Rating: (2522)	Your warning is well taken. That's also why I recommended the hack be used only within the same block and in sequential networks - that keeps everything consistent. jacek d 0_Einstein Here's where things are possibly going to blow your (and other people's) mind! Be warned. This technique can be used in different execution levels (i.e. in different tasks). Therefore data access concurrency can occur. Consistency of data must be assured. Otherwise not only mind will be blown but also whole subtile plan.
	science guy
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