Hi @ESP_alisitsyn, that's great information.

In terms of adding a custom function to mbc_serial_master_send_request, I assume this requires modifying the code for whatever version of ESP-IDF is installed by platformio. And this makes it liable to be overwritten. I'm sure that is manageable but is ...

The PZEM-004T is a popular module for measuring AC voltage, current, power etc. A datasheet can be found here: https://innovatorsguru.com/wp-content/uploads/2019/06/PZEM-004T-V3.0-Datasheet-User-Manual.pdf (and also attached).

The datasheet states: The application layer use the Modbus-RTU protocol ...