Informations about the OpenCM9.04 board and Pypot

Technology
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The Darwin Mini robot uses this board to power and control XL 320 servos http://support.robotis.com/en/product/auxdevice/controller/opencm9.04.htm

Most of resellers say it could drive up to 4 motors, but the Darwin Mini kit uses 16 servos, do you know how is it possible?

This board seems to be connectable to a regular computer through USB, do you know if Pypot supports it?

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OpenCM 9.04 A and B uses large connectors, model C is XL320 equipped.
But apart from a size connector adapter, all 3 boards are able to drive any Dynamixel servos.
In Darwin Mini case, everything is ready to handle the 16 servos included (and the missing 17th for the head) with 2 batteries connected to the power distribution included.

If you’re interested, I’m trying to share my (slow) work on using OpenCM9.04C with pypot.

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I think the resellers are confused. What they mean is that the board has capability for 4 connectors (which by the way could be either the mini one with 2mm pitch used by the XL servos or the 2.54 pitch SPOX used by the AX and MX series, or even mixed). But you can daisy chain as many servos as you want (well not really as many you start getting limited by the power on one bus) - but you should easily be able to drive 4-6 motors. The two factors that drive this limitation are:

  • the current consumption; obviously the connectors and the cables have a maximum current load: for Molex SPOX is about 5A and for the 22AWG cables that come with the AX and MX servos is about 6A; the stall current for one of those servos is between 1A-1.5A so you should be able to drive about 6 on a chain provided not all stall at once. Even in that case the circuit will probably survive for a short period of time - but cables might get warm. Because they are chained the most stress will be on the first cable on and the connector on the board.
  • the length of the chain: the more servos you put on the chain the longer the communication path will be and there is increased risk of communication errors on the bus

All in all you can have easily 6 servos on one connector bus. I had an improved Bioloid that used 4 chains and the two that drove the legs were with 6 servos each – and I didn’t have any issues.