Q: Missing USB "out" endpoint? I am trying to communicate to a USB device which is a laser controller. There is a Windows controller for this device which both writes (turn laser on or off) and reads (get the status of the laser, on or off). Now I want to replicate this in a Linux machine using Python. I was able to successfully read from the device but for some reason I cannot write to it (see this SO question for more details).
I don't have much experience on USB communication, but I have seen somewhere that the IN endpoint is for reading and the OUT endpoint is for writing, and I have noticed that when I do lsusb -v -d <my_device> I get an IN endpoint but there is no OUT endpoint, so this may be the reason of my problem. It looks like a "read only device" (I may be wrong).
My question now is: Is it possible that in Linux I cannot see the OUT endpoint due to some missing driver, or some configuration?
This is the output of lsusb -v -d <my_device>:
Bus 002 Device 010: ID c251:2201 Keil Software, Inc. LASER Driver IJS
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0        64
  idVendor           0xc251 Keil Software, Inc.
  idProduct          0x2201 
  bcdDevice            1.00
  iManufacturer           1 LASER Driver 
  iProduct                2 LASER Driver IJS
  iSerial                 3 0001A0000000
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength       0x0022
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xc0
      Self Powered
    MaxPower              100mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      0 
      bInterfaceProtocol      0 
      iInterface              4 HID
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.00
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      33
          Report Descriptor: (length is 33)
            Item(Global): Usage Page, data= [ 0x00 0xff ] 65280
                            (null)
            Item(Local ): Usage, data= [ 0x01 ] 1
                            (null)
            Item(Main  ): Collection, data= [ 0x01 ] 1
                            Application
            Item(Global): Logical Minimum, data= [ 0x00 ] 0
            Item(Global): Logical Maximum, data= [ 0xff 0x00 ] 255
            Item(Global): Report Size, data= [ 0x08 ] 8
            Item(Global): Report Count, data= [ 0x40 ] 64
            Item(Local ): Usage, data= [ 0x10 ] 16
                            (null)
            Item(Main  ): Input, data= [ 0x02 ] 2
                            Data Variable Absolute No_Wrap Linear
                            Preferred_State No_Null_Position Non_Volatile Bitfield
            Item(Global): Report Count, data= [ 0x40 ] 64
            Item(Local ): Usage, data= [ 0x10 ] 16
                            (null)
            Item(Main  ): Output, data= [ 0x02 ] 2
                            Data Variable Absolute No_Wrap Linear
                            Preferred_State No_Null_Position Non_Volatile Bitfield
            Item(Global): Report Count, data= [ 0x01 ] 1
            Item(Local ): Usage, data= [ 0x01 ] 1
                            (null)
            Item(Main  ): Feature, data= [ 0x02 ] 2
                            Data Variable Absolute No_Wrap Linear
                            Preferred_State No_Null_Position Non_Volatile Bitfield
            Item(Main  ): End Collection, data=none
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               1
can't get device qualifier: Resource temporarily unavailable
can't get debug descriptor: Resource temporarily unavailable
Device Status:     0x0001
  Self Powered


A: Every USB device will accept control commands in the default endpoint, i.e. endpoint #0. This is also a basic requirement for configuring the USB bus, so this doesn't have to be mentioned in the endpoint descriptors, because the endpoint #0 must always exist. The default endpoint is also bi-directional, so a single control transfer can include both a command from the computer to the device and the device's response.
It's quite likely that your laser controller uses the endpoint #0 for commands for the computer to the laser controller. The endpoint #1 is an interrupt endpoint, so it's probably being used by the controller to report any changes in the laser status, or error conditions that must be reacted to in a timely manner (e.g. "laser is overheating").
The laser controller's USB interface uses the HID interface class, because the specification of the HID class includes facilities for miscellaneous and vendor-specific controls.
The interface descriptor of the device has bNumEndpoints as 1, so it confirms there is supposed to be just one endpoint in addition to the always-present control endpoint #0, and you are not missing anything.
