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Ryan Budney
Asked: 2014-08-13 19:32:08 +0800 CST

Probing a USB device, likely a CAN bus

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I have a USB device that is likely a CAN bus and I'm trying to interact with it. I'm wondering if anyone can offer any advice for probing and discovering what services are available. I suppose a good place to start would be to check whether or not its actually a CAN bus. I'm not certain how to do that, though.

Specifically, I'm plugged into the USB interface for a DJI Phantom quad copter. It's a small autonomous drone.

Here's what I have so far:

If I call lsusb with the drone plugged in and powered-up, 

Bus 003 Device 003: ID fff0:0008

is new. lsusb -vvv gives the further details:

Bus 003 Device 003: ID fff0:0008
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            2 Communications
  bDeviceSubClass         0
  bDeviceProtocol         0
  bMaxPacketSize0        64
  idVendor           0xfff0
  idProduct          0x0008
  bcdDevice            1.00
  iManufacturer           1 DJI  CONFIF
  iProduct                2 DJI    CONTROLLER
  iSerial                 3 0001A0000000
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           75
    bNumInterfaces          2
    bConfigurationValue     1
    iConfiguration          0
    bmAttributes         0x80
      (Bus Powered)
    MaxPower              100mA
    Interface Association:
      bLength                 8
      bDescriptorType        11
      bFirstInterface         0
      bInterfaceCount         2
      bFunctionClass          2 Communications
      bFunctionSubClass       2 Abstract (modem)
      bFunctionProtocol       0 None
      iFunction               4 VCOM
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         2 Communications
      bInterfaceSubClass      2 Abstract (modem)
      bInterfaceProtocol      0 None
      iInterface              4 VCOM
      CDC Header:
        bcdCDC               1.10
      CDC Call Management:
        bmCapabilities       0x01
          call management
        bDataInterface          1
      CDC ACM:
        bmCapabilities       0x02
          line coding and serial state
      CDC Union:
        bMasterInterface        0
        bSlaveInterface         1
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0010  1x 16 bytes
        bInterval               2
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        1
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass        10 CDC Data
      bInterfaceSubClass      0 Unused
      bInterfaceProtocol      0
      iInterface              4 VCOM
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x02  EP 2 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x82  EP 2 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               0
Device Status:     0x0000
  (Bus Powered)

if I call udevadm and remove the quad copter's USB cable, I get:

UDEV  [41256.502177] remove   /devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.0/tty/ttyACM0 (tty)
UDEV  [41256.502247] remove   /devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.1 (usb)
UDEV  [41256.502286] remove   /devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.0 (usb)
UDEV  [41256.502346] remove   /devices/pci0000:00/0000:00:14.0/usb3/3-2 (usb)`

and when I plug it in...

`UDEV  [41261.068689] add      /devices/pci0000:00/0000:00:14.0/usb3/3-2 (usb)
UDEV  [41261.070265] add      /devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.0 (usb)
UDEV  [41261.071507] add      /devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.1 (usb)
UDEV  [41261.072843] add      /devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.0/tty/ttyACM0 (tty)

So I try to treat it like a CAN bus interface. It seems okay with the command:

sudo slcand -o -s6 -t hw -S 1000000 /dev/ttyACM0

i.e. no error messages, and 

sudo ip link set up slcan0

creates the network interface slcan0 and ifconfig now shows:

slcan0    Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          UP RUNNING NOARP  MTU:16  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:10
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

From this can anyone tell if this is really a CAN bus I'm interacting with? Or am I walking down the wrong path?

usb

0 Answers