TCA9548A Multiplexer issue with Raspberry


at first Mycodo is for me a masterpiece which makes everything possible great work appreciate it.

I want to build a Climatecontroller for my Mushroom Cultivation Tents. I got for my low amount of know how good progression and Sensors,LCD and Relais working good. Except this damn i2c I/O erno 121 Error.

I connected sht21 Sensor and LCD 16x2 to my TCA9548A what seems to work but not perfectly.

When i turn off my LCD i get no errors but when both i2c devices are running i get this error above. I still get readings from my Sensor but not everytime.

TCA9548A is connected to 5V.

Sorry for my english.

i hope you guys can help me and get me deeper in Microcontrolling.

best regards

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There’s not enough information to determine the issue. IO errors generally mean it’s not able to communicate with the device. Can you provide more detailed information about your wiring and software setup?

Hello Kyle greetings from Germany,

This was my Simple original Setup. I was getting Very much Errors and LCD Display didnt worked. Problem was i had to solder the Pullup on my HTU21D. Didnt know that.

Now LCD and Sensor is working together and errors getting rare.

My new Setup got a 2 Channel Relay inside its working good but i get “Erno 121” more often when i switch the relay module. I want to understand what happens there. Time between measurements and LCD update rate seems to influence this issue.

i have to work soon so i cant provide any new wiring for my new setup. will do soon.

thank you for your help

Heres the new Sketch.

Testet in my Tent. When Relay off no Errors when Relay is on its producing Erno121 for my SHT21(HTU21D) every few Seconds.

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Hello again still have the problem. but i have maybe found out what cause those problems.

maybe its a EMV while switching ac load. my Relais is near to the raspberry. everytime i switch ac error 121 comes up. when i switch the relais without AC its working perfect.

is my relais to close to the raspberry ?

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If it’s a mechanical relay, there is a physical closing of metal connections. When this occurs, there will almost always be a spark, which can cause significant electromagnetic interference to sensitive electronics. If the proximity of your switching relay alone can determine if the Pi experiences issues, I believe you have likely discovered the cause.