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OWI Robotic Arm Edge USB protocol (and sampe code)

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In my last post I have described OWI-007 Robotic Arm parallel port protocol. They have a newer, more advanced model called Robotic Arm Edge:

Robotic Arm Edge

You can use USB interface module to connect it to PC. Unfortunately official software works only under Windows and do not provide API. I have been able to reverse engineer their USB protocol and write sample code which allows to control the arm from Mac and Linux.

This is USB device with vendor id 0x1267 and product id 0. The device is controlled by 3 byte commands sent via USB control transfers. Command format is byte0, byte1, byte2. Each byte controls a group of arm features. All motors could be controlled independently. Most commands start action which is continued until next action is signalled. Byte '00' universally used as stop action.

Bit numbering in byte:

7 (most significant) 6 5 4 3 2 1 0 (least significant)

Meaning of bits in each byte:

Byte 0

First byte controls grip, shoulder and wrist. Bits are assigned as following:

Bit numbers Controls Bit combinations
0,1 Grip 00-do not move, 01-close, 10-open
2,3 Wrist 00-do not move, 01-move up, 10-move down
4,5 Elbow 00-do not move, 01-move up, 10-move down
6,7 Shoulder 00-do not move, 01-move up, 10-move down

Sample commands, activating single motor:

Byte 0 Command
00 stop grip, wrist, elbow and shoulder movement
01 grip close
02 grip open
04 wrist up
08 wrist down
10 elbow up
20 elbow down
40 shoulder up
80 shoulder down

Sample commands activating 2 motors simultaneously:

Byte 0 Command
11 grip close and elbow up
82 grip open and shoulder down


Second byte controls base.
Bits are assigned as following:

Bit numbers Controls Bit combinations
0,1 Base 00-do not move, 01-rotate clockwise, 10-rotate counter-clocwise

Sample commands:

Byte 1 Command
00 stop base rotation
01 rotate base clockwise
02 rotate base counter-clocwise

Byte 2

Third byte controls LED light inside the grip.
Bits are assigned as following:

Bit numbers Controls Bit combinations
0 LED 0-off, 1-on

Sample commands:

Byte 2 Command
00 LED off
01 LED on

Sample source code using libusb:

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <string.h>

#include <libusb-1.0/libusb.h>


#define ARM_VENDOR       0x1267
#define ARM_PRODUCT      0
#define CMD_DATALEN      3

libusb_device * find_arm(libusb_device **devs)
	libusb_device *dev;
	int i = 0;

	while ((dev = devs[i++]) != NULL) {
		struct libusb_device_descriptor desc;
		int r = libusb_get_device_descriptor(dev, &desc);
		if (r < 0) {
			fprintf(stderr, "failed to get device descriptor");
			return NULL;
		if(desc.idVendor == ARM_VENDOR &&
		   desc.idProduct == ARM_PRODUCT)
		    return dev;
	return NULL;

int main(int ac, char **av)
        fprintf(stderr,"Usage: armedgetest CMD0 CMD1 CMD2\n");
        return 1;

    unsigned char cmd[3];

    cmd[0]=(unsigned char)strtol(av[1],NULL,16);
    cmd[1]=(unsigned char)strtol(av[2],NULL,16);
    cmd[2]=(unsigned char)strtol(av[3],NULL,16);
	libusb_device **devs;
    libusb_device *dev;
    struct libusb_device_handle *devh = NULL;
	int r;
	ssize_t cnt;

	r = libusb_init(NULL);
	if (r < 0)
	    fprintf(stderr, "failed to initialize libusb\n");
	    return r;

	cnt = libusb_get_device_list(NULL, &devs);
	if (cnt < 0)
		return (int) cnt;
	    fprintf(stderr, "Robot Arm not found\n");
	    return -1;

    r = libusb_open(dev,&devh);
	    fprintf(stderr, "Error opening device\n");
       	    libusb_free_device_list(devs, 1);
	    return -1;

    fprintf(stderr, "Sending %02X %02X %02X\n",

    int actual_length=-1;
    r = libusb_control_transfer(devh,
                                0x40, //uint8_t 	bmRequestType,
                                6, //uint8_t 	bRequest,
                                0x100, //uint16_t 	wValue,
                                0,//uint16_t 	wIndex,
    if(!(r == 0 && actual_length >= CMD_DATALEN))
        fprintf(stderr, "Write err %d. len=%d\n",r,actual_length);
	libusb_free_device_list(devs, 1);

    fprintf(stderr, "Done\n");
	return 0;

Sample usage:
./armedgetest 00 00 01
sleep 3
./armedgetest 00 00 00

will turn the light on for 3 seconds and then would turn it off. It have been tested on MacOS X, but should work on Linux as well.
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[User Picture]
On January 15th, 2011 11:38 pm (UTC), polwart commented:
Thanks for the details of the protocol
I'm using controlling this arm as a way to polish my Python skills, so your details of the USB structure were very helpful. I've got the basics working in Python and will tidy up the code and post it on line for others who may be interested in the next day or so. I've created a blog for the purpose here: http://python-poly.blogspot.com
[User Picture]
On January 17th, 2011 01:14 am (UTC), notbrainsurgery replied:
Re: Thanks for the details of the protocol
good to hear this! Perhaps you can set up a project at google code and post both my C and your Python code there under open-source license?
[User Picture]
On January 27th, 2011 07:13 pm (UTC), polwart replied:
Re: Thanks for the details of the protocol
Notbrainsurgery, I'm not really familiar with the world of "google code" nor is my code really that sophisticated that it needs a "project". All my code is on my blog, and I've no issues with others using it for non-commercial purposes. I've already got a link back here from the blog - and to the C# code on linux questions.

If someone wants to create a project I'd happily contribute my python code, but my main aim is to get my python skills up to practical levels - so the code I write might not be much interest to others beyond the basic control functions. I'll be adding an upgrade later tonight or tomorrow.
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