ArbotiX-M partially dead

[Hugh]

Hi,
I think I killed the Dynamixel serial port on my ArbotiX-M when I accidentally shorted the bottom of a connected power-hub.

The motors seem ok using a different board, and the usb communications to the board still works fine and there is 12v on the Dynamixel headers, but It can't detect/communicate with the motors anymore.

Is there any way to work round or repair this, or should I just use this for non-dynamixel projects now.

Thank,
Hugh

[jwatte]

Some TTL bus interface boards use a buffer IC to go between the microcontroller and the bus, and sometimes it's possible to replace that IC.
However, looking at the Arbotix-M, it looks like the only interface chip is between the MCU and the Xbee (and that's probably a regulator) so I think that won't work.
You probably should make sure that you didn't screw up the servo itself, if the servo was connected while the accidental short happened.

In general, building electronics that works fine when everything is correctly hooked up is one challenge (because functionality, cost, weight, space ... they all interact.)
Building electronics that is robust to accidents, and yet doesn't degrade the signal quality, is much harder, and generally generates bigger circuit boards with more components and higher cost and weight :-(

[Hugh]

The actuators seem ok. When it happened, I though "you idiot!". But maybe not toooo much damage and a good lesson.
Mind you, adding a 1c piece of plastic on the back of the connectors would make them a better design. I notice the ones with the built-in DC barrel jack have a plastic wrap, and these connectors are just as "live", so similar precautions are probably prudent.
Certainly cheaper that redesigning the controller board.

[jwatte]

adding a 1c piece of plastic on the back of the connectors

I'm at the point where I tape the backside of circuit boards when I get or make them, because I've placed them on a metal object/surface one too many times ...
I bought some acrylic conformal coating, but it requires too many sprays and too many dries to be effective, so the tape is much faster.
The other thing you can do (as circuit designer) is to add output resistors and perhaps even PTC fuses on all public-facing pins, and Zener or TVS diodes to make sure the voltage is never too much, and perhaps some capacitance with high ESR on wires that aren't high data rate, to avoid too much inductive swing when connecting (certainly on power inputs!)
Also, a power MOSFET to prevent reverse polarity makes for a more robust board (and gets rid of diode losses,) but those add significant cost if you're expecting to pull some current.

20 cents per pin for resistor and PTC: http://www.digikey.com/product-detai...70-2-ND/588491
Antother 5 cents per pin for TVS: http://www.digikey.com/product-detai...FTR-ND/4898755
40 cents per power input (common ground systems need P-channel): http://www.digikey.com/product-detai...-13-ND/6187320

Another 10 cents per power input (overvoltage protection): http://www.digikey.com/product-detai...DITR-ND/768796
Another 40 cents per power input (overcurrent protection, combos well with TVS): http://www.digikey.com/product-detai...6TB-ND/1211325
Another 10 cents per power input (capacitor): http://www.digikey.com/product-detai...13-2-ND/589854

Of course, in quantity 1, expect to pay 5x-10x as much for the components ...

In addition to the extra cost, the board space to protect 28 I/O pins and 1 or 2 power pins is not insignificant.