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Thread: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

  1. Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    Hi All,
    To make my robot more powerfull I'm trying to combine my MX-28AR(RS-485) servos with AX-12(TTL).
    Is it possible to use 4 pin Dynamixel(RS-485) and 3 pin Dynamixel (TTL) simultaneously on Arbotix Pro?
    Is there any change I could make to do that?

    Regards,

    wiweet
    Last edited by wiweet; 05-04-2015 at 12:52 PM.

  2. #2

    Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    What happens if you just plug both of them in at the same time?
    Can you scan them both? It seems like it should work (given the protocol involved.)

  3. Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    Well actually, right after posting my question I did try to connect both of the RS-485 and TTL Dynamixel at the same time. My finding is that if the selector jumper for TTL or RS485 on the Arbotix Pro board positioned at TTL we can scan/found both of the RS-485 and TTL Dynamixel, but when it's positioned at 485, only the RS-485 Dynamixel found, not the TTL. Is it the right behavior of the Arbotix Pro, I mean, when the jumper is positioned at TTL we can scan/found both of the RS-485 and the TTL Dynamixel?Is it safe for the Board and the Dynamixel to use TTL mode on the Arbotix Pro to communicate both to RS-485 and TTL Dynamixel?

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  4. #4
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    Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    If you are always getting a response from the MX-28AR, then the RS-485 transceiver would appear to always be active. If so, then the jumper is probably only disabling the RX of the TTL buffers. But if they have all the parts for both buss types already populated and the busses/connectors isolated, why bother with the jumper and just let anything connected be found? Or the single sided swing of the TTL bus is sufficient to trick the MX-28AR RS-485 transceivers, which is probably not great for long term board life.
    Last edited by tician; 05-04-2015 at 04:07 PM.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  5. #5

    Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    If we had the schematics, we could tell for sure...

  6. Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    As far as I know the schematic is posted on https://github.com/Interbotix/Arboti...aster/hardware , eventhough it's writen Arbotix Pro V1.1 and the board I had is V1.0, I don't know is there any significant differences between those version.
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    Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    Yep, RS-485 transceiver is always active and driving the 485_D- signal whenever TTL_DATA is transmitting. If the TTL_DATA signal is sufficiently close to being complementary to the 485_D- signal (i.e. equivalent to the 485_D+ signal), then there is the chance that the voltage difference between DATA and 485_D- are sufficient to be received by the servos. Not sure if the MX-28AR have termination resistors, but they may be playing a role in it working (only supposed to be one on each end of a chain). Not going to be a good thing to continue doing if you want the servos and Arbotix Pro to last long.

    Better design would have been to cut the trace connecting the 3-pin comms to the 4-pin comms to keep them completely isolated yet operating on the same UART through their respective transceivers/buffers, just like the CM-700 does.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  8. Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    Ok. What if I disconnect the 3rd pin from the 4 pin servo cable and take the 485_D+ directly from the MAX3443 output(pin number 1 of the jumper header/JP6 in this case)?So the 4pin servo is getting the 485_D+ and 485_D- directly from MAX3443.Is it a possible and safer alternative?
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  9. Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    Quote Originally Posted by wiweet View Post
    As far as I know the schematic is posted on https://github.com/Interbotix/Arboti...aster/hardware , eventhough it's writen Arbotix Pro V1.1 and the board I had is V1.0, I don't know is there any significant differences between those version.
    Any chance you could post the rest of the schematic for those that don't use eagle?

  10. #10

    Re: Mixing Dynamixel Servo (4 pin & 3pin) on Arbotix Pro

    I did create a PDF from the schematic, which I could post, but I am hesitant to do so, without Trossen saying it is OK.
    Should be fine as the stuff in the project is shown as open source...

    Note: I personally use Diptrace to do any designs that I make for myself, but did purchase a light weight Eagle license (6.3 light) to allow me to do stuff including converting Eagle component libraries into Diptrace libraries.

    But before that I installed the Eagle free version as a way to look at designs like this. You can download it from: http://www.cadsoftusa.com/download-eagle/freeware/

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