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I'm attempting to put an HC-05 module into command mode in order to change the PIN and default baud rate etc. According to the datasheet, PIO11 needs to be high to do this, so I connect it to 3.3v, remove power and power it back up.

Using an FT232 interface and a level converter, I send the AT command to it via Putty. I can observe the command on the RX pin of the HC-05 module but I don't see anything come back. I've tried various combinations of baud rates and flow control settings.

For the record, I can pair with the module and receive data sent to it in my Putty window via the FT232/level converter.

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I got there in the end with this. Bear in mind the following conditions when trying to send AT commands to the HC-05:

  • The default baud rate on my module was 9600, not 38400 as described in certain places
  • If you make PIO11 (key) high (3.3v) while the module is on, it will enter AT command mode but will retain the default baud rate
  • If you power the module up with PIO11 high, it will enter AT command mode but the baud rate will be set to 38400
  • All AT commands must end with Carriage Return and Line Feed (\r\n)

The key bit for me was sending AT commands CR & LF enabled on transmit.

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