# MQTT topics creates loop with Serial Port
**URL:** https://discourse.nodered.org/t/mqtt-topics-creates-loop-with-serial-port/96914
**Category:** General
**Tags:** mqtt, node-red-node-serialport
**Created:** [5 May 2025 21:06 UTC](https://discourse.nodered.org/t/mqtt-topics-creates-loop-with-serial-port/96914 "2025-05-05T21:06:03Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@WannabeMaker](https://discourse.nodered.org/u/WannabeMaker)
#### Post date: [5 May 2025 21:06 UTC](https://discourse.nodered.org/t/mqtt-topics-creates-loop-with-serial-port/96914/1 "2025-05-05T21:06:03Z")
Hello,
I have question regarding how to deal with MQTT topics.
I have the following flow that I use to control a sensirion MFC5500. The flow works well without bug. The one problem that I have is when I want to do read the sensor value, I inadvertently created an endless loop.
I trigger a measurement by sending a payload ( "-") on the topic mfc/A01/read. The sensor returns its value as a new payload on the same topic; which starts an infinite loop. In itself it is not the worst unforeseen issue since I need to get measurement at regular intervals, but I would rather have a little bit more control as to when those happen.
What would be the best practice to deal with a situation like this? Is it to check the type of payload (ex. string vs. number) and ignore messages that are not the right type? Is it to create a different topic for the sensor data in a separate branch?
In this specific instance I have multiple type of sensors and it is convenient for me to segregate them by type (ex. mfc, temp, pressure, etc..) and then using a different topic to identify the specific sensor (ex. A01, H03, etc...).
I know their might not be one right solution, I am just curious what the hive mind thinks.
Thank you in advance.
W.
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### Author:  [@jbudd](https://discourse.nodered.org/u/jbudd)
#### Post date: [5 May 2025 22:26 UTC](https://discourse.nodered.org/t/mqtt-topics-creates-loop-with-serial-port/96914/2 "2025-05-05T22:26:00Z")
Personally I would arrange for requests sent to the sensor device to have a different topic from replies. For example `mfc/A01/read` and `mfc/A01/reply`.
Then your mqtt-in node could subscribe to `mfc/+/reply` (/+/ means any single word in the topic, thus it matches `mfc/A01/reply` and `mfc/H03/reply`, etc).
Failing that, put a switch node after your mqtt-in to discard those messages where msg.payload === "-"
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### Author:  [@system](https://discourse.nodered.org/u/system)
#### Post date: [3 August 2025 22:26 UTC](https://discourse.nodered.org/t/mqtt-topics-creates-loop-with-serial-port/96914/3 "2025-08-03T22:26:09Z")
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