POST No. 2407582
not getting a UART Rx response from turtlebot motor
2019-09-07 10:31:23 jwillick
I am trying to communicate over UART with the motor XL430-W250-T from a turtlebot 3.  I am using something other than the ST micro processor on the OpenCR board (I tested that and it works so I know my motors are good)  I am using a Qualcomm 845 processor for the UART (Robotics RB3 platform).  I have a different board which has the dual buffer chip for combining the Tx and Rx signals.  I looked on the scope and the Tx is getting through to the dynamixel and looks clean.

I am setting the baud rate to 57600 and using protocol 2. 

Then I set the UART direction enable pin (for the dual buffer part) to high for the transmit direction

Then I am sending this bit stream: 


which I got from here:  http://emanual.robotis.com/docs/en/dxl/protocol2/

Then I am reversing the dual buffer direction about 500us after and waiting for a response and not seeing anything on the scope.

My question is, am I sending the correct bit stream?  Is there something else I am missing first?

thank you!


2019-09-07 10:31:23
2019-09-10 06:26:05 Kurt Eckhardt
Normal Caveat that @willson can answer this a lot better than I can.

But I believe your stream is wrong. In particular your CRC is wrong. 
Note: I did not validate it, but it looks like you simply copied the first example in the e-manual you linked to:
0xFF 0xFF 0xFD 0x00 0x01 0x03 0x00 0x01 0x19 0x4E

And then you changed from ID=1 to broadcast ID=0xfe  but you did not change the CRC. Which is in the 2nd example
0xFF 0xFF 0xFD 0x00 0xFE 0x03 0x00 0x01 0x31 0x42

Notice how the last two bytes changed.   Note there is a section in that manual which links you off to CRC calculation code
2019-09-10 06:26:05
2019-09-11 03:29:42 Jonathan
cool thanks, I'll give that a shot
2019-09-11 03:29:42
2019-09-11 12:53:51 Jonathan

found the problem, the dynamixel I had was programmed for a different baud rate and ID.  I ordered the R2D2 dongle and used the R+ Manager software to change it and now I can communicate with it, I see receive data coming back over my UART interface now.

2019-09-11 12:53:51
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