Connection Problem HC-05 with ELM327

Projects Networking, Protocols, and Devices
1

Hello everyone,

I am trying to connect a HC-05 BT Module with a ELM327. This was done before by a lot of people and problems with it have been discussed here or here as well. However none of the results I found helped me further, so here we go:

Basically I could not sucessfully perform the command

AT+LINK=001D,A5,68988B

Here a little background on my setup:

I want to use a Arduino later on, for now I am having a TTL-USB Adapter connected to a HC-05. On power up and all the way PIN 34 is on HIGH to boot in AT-Mode; I am using 5V as VCC.

The ELM37 is right now in kind of a "bench mode", that means I have the PINS 4, 5 & 6 connected to GND and PIN 16 at 12 V on a switching power supply.

To perform the connection I basically performed the following inputs (as posted similar by Power_Broker) on the HC-05:

AT+ORGL 
AT+uart=38400,1,0
AT+ROLE=1 						
AT+CMODE=0						
AT+PSWD=1234     
AT+BIND=1D,A5,68988B
AT+INIT 
AT+PAIR=1D,A5,68988B,20
AT+CMODE=1
AT+LINK=1D,A5,68988B

I tried different commands in different orders as well, for the AT+LINK I always get:

FAIL
+INQ:1D:A5:68988B,1F00,7FFF
OK

So first it fails but in theory it should be fine, right?

After doing that in AT-Mode I reset the HC-05 witout PIN34 HIGH. The LED starts a combination of long and short blinking pattern. To figure out whats going on I press the little button on the HC-05 while sending a "AT+STATE=?"; I get the following response:

+STATE:INQUIRING
OK
+STATE:CONNECTING
OK
+STATE:CONNECTING
OK
+STATE:INQUIRING
OK
+STATE:INQUIRING
OK
+STATE:INQUIRING
OK
+STATE:CONNECTING
OK

So it seems the module is trying to connect but does not succeed.

Things I already checked to find the error:

  • I tried the ELM327 in a real car, not only the bench
  • I connected sugessfully a android device to the ELM327 (bench and car), was able to get response with a BT-Client and the command "AT Z" AND disconnected the device afterward again
  • I could connect the android device to the HC-05; the blinking of the LED changed to two short blinks every one or two secounds. However this is only true when connecting from Android to the HC-05. When I try it the other way around in the same way I try to connect the ELM, I have the exact same problem (always connecting, never actually connected). When I actively select this device in the BT-Client the connection finally succed an I could send data back and forth.
  • I set the password again with "AT+PSWD=1234" and tried "AT+PSWD="1234"" as well
  • I tried to use the Adress "001D,A5,68988B" instead of "1D,A5,68988B" (I read somewhere it need to have a 4,2,6 pattern)

I could not shake the feeling that the problem has either to do with a wrong adress or a wrong authentification...

So thats all I could think of right now... :slight_smile:

Ps: When I use a wrong adress for the ELM on purpose I could not even Pair. However when I use a wrong password like "0000" it is the exact same behavior than with the correct one. So could it be a mistake of how the password is set?

2

Hey guys,

so after hours of work I tried as a last resort to solve this problem with money, means: I ordered a new HC-05. And what could I say: First try and its working like a charm. Son of a mother!

So maybe someone is wondering what module I had on hand: One major difference with the new one is, that the input is now only accepted using capital letter ("AT" instead of "at").