HC-05 Module Does not Respond to Some AT commands

Using Arduino Networking, Protocols, and Devices
1

The HC-05 modules that I have purchased from HiLetGo do not work properly. I have gotten both into command mode, only to discover that I cannot get the chips to respond to any commands unless their communication leads are wired incorrectly (see wiring break down below) . When in AT mode, the chips do not respond to the PAIR, INIT, RMAAD, and INQM do not work at all. The bind command will not respond at all unless the last six characters in the xxxx,xx,xxxxxx parameter are all letters, and even then the bind parameter will remain in the default. The INQ command appears to put these chips into the inquiring state, though it will not print ok to the screen.

I have reset these chips multiple times, and still, they refuse to connect or change from their default bind state.

Wiring break Down chip -> board:
GND -> GND
TX -> TX
RX -> RX
VCC -> VCC

I know the TX and RX are wired up in reverse, I have no idea why but this is the only way to get these chips to respond to any AT commands.

2

In this HC05 page Martyn Currey talks about the AT mode issues with different HC05s and the solutions. This information helped me. I discovered that if I hold the button down with a small alligator clip, power the HC05 and leave the clip holding the button, I have full access to the AT commands (on the 8 or so HC05 modules that I have). And some work by connection EN to 3.3V and powering the HC05.

3

The Tx>Tx and Rx>Rx is correct if you are using a blank sketch and the monitor input is directly passed through the usb converter chip to the Rx Tx pins.

4

I have put a clamp on to hold down the button, but the chip is still acting just as it has been. I still cannot use a good chunk of the AT commands. I do not have the equipment on hand to try powering pin 34. Will try that idea out as soon as possible.

5

Did you try connecting the EN pin to 3.3V before powering the HC05 and leaving EN connected to 3.3V after applying power? Make sure that it is 3.3V not 5V as 5V to any of the pins (except Vcc) can damage the HC05.

6

Yes, doing this makes the chip acknowledge all at+bind= commands, though it does not change the bind parameter. Most of the other AT commands do not work.

7

I do not know what to tell you then. I got mine accept all the commands, that I tried, by using one of the methods that I described (alligator clip on button or 3.3V to EN).

Did you also read of the different modules that are out there?
From the Martyn Currie page that I linked before:

There are also modules that use the same breakout board but have different board markings such as the fc-114 modules:
HC-05 FC-114 and HC-06 FC-114. First Look
HC-05 FC-114 and HC-06 FC-114. Part 2 – Basic AT commands
HC-05 FC-114 and HC-06 FC-114. Part 3 – Master Mode and Auto Connect‘

8

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