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

  1. #11

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

    Wow, that's a dangerous configuration! If you turn on RS485 while you have a TTL servo connected, you'll potentially push 12V into the TTL input of the TTL servos.

    Even when the switch is at "TTL," the RS485 servos may push high voltage into the TTL bus. I'd want at least a resistor and a low-capacitance Zener across the TTL bus to avoid that problem (resistor between 4P1 and DXL1; Zener across the TTL part of DXL1 and ground.)

    The reason reception works anyway is likely that the level translator from the TTL side will pull the AND gate low, so it doesn't matter if the RS485 will detect it or not.

    Actually, a better design would have been to push TTL out to TTL, and RS485 out to RS485, and then just use the AND gate for input detection, with no cross-coupling of them. It would be safer, less components, and more robust IMO. (Am I missing something here?)

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

    Quote Originally Posted by jwatte View Post
    Actually, a better design would have been to push TTL out to TTL, and RS485 out to RS485, and then just use the AND gate for input detection, with no cross-coupling of them. It would be safer, less components, and more robust IMO. (Am I missing something here?)
    Which is basically what the CM-700 has set up. TX goes out to both the RS-485 transceiver and TTL buffers then to their respective connectors with electrically separate dxl busses. RX comes in through RS-485 and TTL buffers to another gate/buffer then on to the microcontroller.

    The Arbotix Pro inherited from the CM-730 and not the CM-700. The CM-730 used an either/or selectable version because it could only have one type of connector populated at a time using a single 4-pin footprint for all the connectors (came with 3-pin by default, but can be desoldered to install 4-pin connectors). Having both connectors populated without separating them is not a great idea.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

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

    Quote Originally Posted by tician View Post
    Which is basically what the CM-700 has set up. TX goes out to both the RS-485 transceiver and TTL buffers then to their respective connectors with electrically separate dxl busses. RX comes in through RS-485 and TTL buffers to another gate/buffer then on to the microcontroller.

    The Arbotix Pro inherited from the CM-730 and not the CM-700. The CM-730 used an either/or selectable version because it could only have one type of connector populated at a time using a single 4-pin footprint for all the connectors (came with 3-pin by default, but can be desoldered to install 4-pin connectors). Having both connectors populated without separating them is not a great idea.
    As for your statement
    Quote Originally Posted by tician View Post
    ....
    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.
    , if I do a little modification to the wiring of the 4pin and 3 pin like shown on the picture below, which is cut the trace connecting the 3-pin comms to the 4-pin comms and then connect the 3 pin comms to TTL_DATA directly. (with the jumper selector JP6 shorted between DATA and 485_D+) is safe enough?so the 3 pin only connected to TTL_DATA and never connected to 485_D+. Is this what you mean to isolate the 3-pin comms to the 4-pin comms?
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by wiweet; 05-05-2015 at 12:15 PM.

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

    Quote Originally Posted by KurtEck View Post
    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/
    I actually thought I had included a PDF of the entire schematic, that was my intention anyway. It's open source under GNU GPL v3 as shown here: https://github.com/Interbotix/Arboti...master/LICENSE So please feel free to do whatever you like with it.

    I have a complete redesign of the TTL / RS-485 circuitry in mind. The current design is nearly an identical copy of the CM730 schematic, which certainly leaves some room for improvement.

    In it's current form, the design intent is NOT to use both RS-485 and TTL servos simultaneously,
    which is why we jumpered it in hopes that users would see it as a 'one or the other' type of option. Production boards don't have the 4P headers populated, and we put 'blank plugs' in the Beta board headers to indicate they were not being used in the beta. Keep in mind we're not selling the boards separately yet, so the intended use is only with the AX-12s in the HROS1 Beta kit.

  5. #15

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

    Thanks Andrew,

    That is what I assumed.

    ArbotixProSchematic.pdf

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