MKR 485 stops working when Serial USB disconnected

Official Hardware MKR Family MKR GSM 1400
1

Greetings,

I've been using the MKR family to read various sensors via ModBus and uploading that sensor data to a Middleware via MQTT.
I'm using both the MKR 1400 GSM Arduino for some applications, and the MKR Zero with an ethernet shield for others.

The problem I'm facing is related to interfacing the 485 shield with the MKR GSM1400. For some reason, I can only read data from ModBus sensors if the MKRGSM has a USB connection, even though the MKR Ethernet shield can works as a power supply due to it's buck converter. I still have communications with the mqtt broker, as I can see payloads incoming from this device.

This is even more strange considering that, the exact same code for reading ModBus slaves using a MKR Zero with an ethernet shield works without any issue.

So, this must be some kind of hardware issue.

Thanks for your support.

Lourenço.

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