Problems connecting reliably using HC-05 (as bluetooth master)

Question

I currently try to connect an Arduino Micro/Leonardo/32u4 to an ELM327 Bluetooth dongle using a HC-05 Bluetooth shield from iTeadStudio. I also have similar problems trying to connect to a bluetooth dongle (with added serial connection) on my PC.

The basic AT-command communication works, but I haven't found out yet why AT+PAIR or AT+LINK are failing and what should be done to avoid this (I've done dozens of Serial Monitor command iterations). Until now I haven't found a command sequence that reliably connects. What I've tried so far (from different documentation and example codes):

Check general AT command set:

AT
> OK

Reset to default values:

AT+ORGL
> OK

Set master mode and check it:

AT+ROLE=1
> OK
AT+ROLE?
> +ROLE:1
> OK

Connect only to the specified bluetooth address:

AT+CMODE=0
> OK

Reset and initialize:

AT+RESET
> OK
AT+INIT
> OK

Configure inquiring mode:

AT+INQM=1,9,48
> OK

Inquire:

AT+INQ
> +INQ:12:34:567890:1F1F,7FFF
> OK
AT+STATE?
> +STATE:INQUIRING

Try to stop inquiring:

AT+INQC
> OK
AT+STATE?
> +STATE:INQUIRING
> OK

Reset and initialize again (otherwise we can't leave the inquiring state):

AT+RESET
> OK
AT+INIT
> OK
AT+STATE?
> +STATE:INITIALIZED
> OK

Try to pair with the found device (my PC asks for the 1234-password and creates a serial port):

AT+PAIR=12,34,567890,20
> OK
AT+STATE?
> +STATE:PAIRED

Try to connect

AT+BIND=12,34,567890
> OK
AT+LINK=12,34,567890
> FAIL
...

Does someone have some hints for finding the right sequence?

Answer 1

I am using a "BIND" between the PAIR and LINK:

AT+PAIR=98D3,31,FC2D54,5
AT+BIND=98D3,31,FC2D54
AT+LINK=98D3,31,FC2D54

That seems to work for me.

See the docs here:

ftp://imall.iteadstudio.com/Modules/IM120723009/DS_IM120723009.pdf

Answer 2

It looks like I now have a slight understanding of what happens and how to get the HC-05 working.

Be sure to have the CMD-DAT-switch set to CMD before turning on the HC-05.

Command mode (configuration)

Check general AT command set:

AT
> OK

Reset to default values (sets, among others, ROLE=0 and CMODE=1):

AT+ORGL
> OK

Set master mode and check it:

AT+ROLE=1
> OK
AT+ROLE?
> +ROLE:1
> OK

Reset and initialize (reset aborts, e.g. the inquiring state - quickly flashing LED; without initializing you would get ERROR(16)):

AT+RESET
> OK
AT+INIT
> OK

Now the LED should slowly flash. Optionally: set password of the target device:

AT+PSWD=1234
> OK

Show count of all authenticated devices in pair list:

AT+ADCN?
> +ADCN:7
> OK

If the value is larger than 0, delete all authenticated devices in pair list:

AT+RMAAD
> OK

Now pair with the desired device (the last 20 mean 20s):

AT+PAIR=12,34,567890,20
> OK

The LED starts flashing one time with a longer pause. Set the desired device:

AT+BIND=12,34,567890
> OK

Now link (e.g. when connecting to the ELM327; the PC will create another new port after having requested the password from the user):

AT+LINK=12,34,567890
> OK

The LED starts flashing two times with a longer pause. After linking, it might be that either the HC-05 switches to data-mode or continues in command-mode (haven't found out when it does one or the other).

Command mode (auto connect)

Now switch off the HC-05 and keep the CMD-DAT-switch to CMD. Switch it on again. It should automatically connect to the previously connected device. To switch to the data mode, first disconnect...

AT+DISC
> +DISC:SUCCESS
> OK

... and reconnect.

AT+LINK=12,34,567890
> OK

Now the connection seems to have switched to data mode.

Data Mode

Now switch off the HC-05 and move the CMD-DAT-switch to DAT. Turn on the HC-05. It automatically will try to connect to the previously connected device - the LED should flash two times with longer pause - and you don't have to mess with HC-05-AT commands any more.