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Thread: Cable length limit for Dynamixel AX-12A?

  1. #1

    Question Cable length limit for Dynamixel AX-12A?

    Hi!

    I have a PhantomX Reactor Robot Arm Kit and am trying to control it with a Master/Slave system. The master arm is driven by the same Dynamixel AX-12A servos as the Slave arm and works fine. However, because the cables are so short, it's hard to be able to stand operate the robot arm which will be on the floor. The longest cables I was able to purchase is 250mm and when connecting 2 with a hub, I'm able to get the master/slave 500mm apart. I tried creating a 1M cable by splicing / soldering a cable with 20AWG silicone stranded wire but it doesn't work with that length. The cable works ok using a meter, but the signals don't go through.

    Is there a way to extend the distance between servos? Perhaps with a powered hub or something?

    Any help is greatly appreciated!

    Thanks,
    Gary

  2. #2
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    Re: Cable length limit for Dynamixel AX-12A?

    Are you using two arbotix-m controllers, one in each arm? If so, just use the FTDI/XBee UART on each of the arbotix-m to make the two boards communicate instead of sharing a single DXL buss between both the arms.
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  3. #3

    Re: Cable length limit for Dynamixel AX-12A?

    Thanks for the reply. Right now I have them all on the same DXL bus and driven by one program. Loop basically gets all the servo positions of the master and then does a single dxlSyncWritePosition on the slave. I'm an noob at this - how would I have two arbotix's talking to one another? And would that be fast enough?

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    Re: Cable length limit for Dynamixel AX-12A?

    Basically, you have each arm with its own wiring, arbotix-m, and power supply. They are connected together with three wires: RXD, TXD, and GND of the 6-pin FTDI header. RXD of one arbotix goes to TXD of the other, and vice versa. GND connect together to provide a common reference voltage.

    One arm is designated as master and programmed with a sketch to poll servo present_position values then send them over the FTDI UART to the slave arm(s). The other arm is programmed with a sketch to receive servo goal positions over the FTDI UART then send a sync_write packet to its servos. The FTDI UART can be accessed as 'Serial' (UART0) while the DXL buss is accessed in the ax12 library using low-level registers of UART1 instead of the 'Serial1' interface. Basically, call 'Serial.begin(some_matching_baudrate)' on both the master and slave, then they should be able to send data back and forth quite easily using 'Serial.read()' and 'Serial.write()'.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
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  5. #5

    Re: Cable length limit for Dynamixel AX-12A?

    Thanks for the idea. I'll need to get another Arbotix-M to try that but I have an Arduino Mega 2560 and a USB2AX on the way. It sounds like I might be able to do something similar with those perhaps, right? the Mega should have multiple serial ports and with the USB2AX I may be able to use a laptop to act as the 2nd controller?

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    Re: Cable length limit for Dynamixel AX-12A?

    I was assuming you already had an arbotix-m from each arm kit. It is possible to control both arms with a single DXL buss, but power should not be shared between the two using the 3-pin cables. Give each arm its own 12V power supply capable of at least 5A via arbotix-m and/or powered hub, then connect the two arms using a modified DXL 3-pin cable that only connects the GND and TTL/DXL pins to share communications between all the servos without any significant interference from power transmission. Since there is no significant current passing through the GND and TTL/DXL wires, they can be much smaller diameter than 20AWG. The connector wiring/pinout is on the robotis support site for the AX-12, but be very sure not to swap wires/signals or you can damage the servos.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  7. #7

    Re: Cable length limit for Dynamixel AX-12A?

    Thanks again for the input. I'll give t his a try. Interestingly, I was able to use 5 servos / hubs to connect 6 250mm wires together and it worked fine. Perhaps it was a problem with the cable I soldered together that caused my first attempt at a long cable to fail.

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