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This is a Ruby port of the HID API from Signal 11.

I am not associated with Signal 11.

More specifically, it is a port of the "libusb" version of the HID API. I took creative liberty where I needed to and basically just sought to make it work uniformly. The gem relies on the libusb.

I know there are at least two other projects that were meant to bring an HID API to the Ruby world. However, one of them is a C plugin (no real problem, just not Ruby) and the other is an FFI wrapper around the original HID API with a few missing components. I didn't see any reason to bring FFI into it when the end result is something fairly simple.

The entire library basically consists of the HIDAPI::Engine and the HIDAPI::Device classes. The HIDAPI module maintains an instance of the HIDAPI::Engine and maps missing methods to the engine. So basically HIDAPI.enumerate is the same as HIDAPI.engine.enumerate where the engine method creates an HIDAPI::Engine on the first call. The HIDAPI::Engine class is used to enumerate and retrieve devices, while the HIDAPI::Device class is used for everything else.

The original source included internationalization. I have not included that (yet), but the HIDAPI::Language class has been defined and the i18n is required, even though we aren't using it yet.


Add this line to your application's Gemfile:

gem 'hidapi'

And then execute:

$ bundle

Or install it yourself as:

$ gem install hidapi


Basic usage would be as follows.

my_dev = HIDAPI::open(0x4d4d, 0xc0c0)
my_dev.write 0x01, 0x02, 0x03, 0x04, 0x05
my_dev.write [ 0x01, 0x02, 0x03, 0x04, 0x05 ]
my_dev.write "\x01\x02\x03\x04\x05"
input =

The write method takes data in any of the 3 forms shown above. Individual arguments, an array of arguments, or a string of arguments. Internally the first two are converted into the 3rd form using pack("C*"). If you have a custom data set your are sending, such as 16 or 32 bit values, then you will likely want to pack the string yourself to prevent issues.

The read method returns a packed string from the device. For instance it may return "\x10\x01\x00". Your application needs to know how to handle the values returned.

There are multiple methods to open a device.

vendor_id = 0x4d4d
product_id = 0xc0c0
serial_number = '123456'
bus_number = 1
device_address = 12
interface = 0
dev_hidapi_path = "/dev/hidapi/my-dev@1-2.3"
dev_raw_path = "/dev/bus/usb/001/00c"

# open with just the vendor and product id.  first match is returned.
my_dev = HIDAPI::open(vendor_id, product_id)

# extend that by also including the serial number. first match is returned, 
# but this time it should definitely be unique
my_dev = HIDAPI::open(vendor_id, product_id, serial_number)

# (linux only) open the device using the hidapi path.
my_dev = HIDAPI::open_path(dev_hidapi_path)

# (linux only) open the device using the raw dev path.
my_dev = HIDAPI::open_path(dev_raw_path)

# open the device using the BUS:ADDRESS:INTERFACE path.
# the components of the path must be in hexadecimal.
my_dev = HIDAPI::open_path("#{bus_number.to_s(16)}:#{device_address.to_s(16)}:#{interface.to_s(16)}")

Because USB is hot-pluggable, you may want to avoid the open_path method unless your device is guaranteed to be plugged into the same port all the time. For instance, a device plugged into a port inside the computer case. Devices plugged into external ports, or hubs, are not necessarily good candidates for open_path because they can be unplugged and plugged into a different port at any time. A device at "001:00c:00" may be at "001:00b:00" the next time because the user swapped the plug into another port.

If you will only have one instance of the device plugged in, it is best to use the open method with the vendor_id and product_id of the device. If you have multiple instances and they each have unique serial numbers, then you would want to use the open method with the vendor_id, product_id, and serial_number. If you have multiple instances with the same serial number (because it is hardcoded into the firmware for example), then you will need to use dedicated ports and open_path.

In order to use a USB device in Linux, udev needs to grant access to the user running the application. If run as root, then it should just work. However, you'd be running it as root. A better option is to have udev grant the appropriate permissions.

In order to use a USB device in OS X, the system needs a kernel extension telling the OS not to map the device to its own HID drivers.

In order to use a USB device in Windows, the system needs the WinUSB driver installed for the device. Please use the Zadig tool to install the driver for your device. Even then Windows seems quirky with libusb. The biggest problem I have noticed is the inability to close a device without exiting the program. I haven't thoroughly investigated it yet because Windows has not been my primary development environment. I would appreciate any feedback related to this issue.

The HIDAPI::SetupTaskHelper handles Linux and OS X situations. The gem includes a rake task setup_hid_device that calls this class. You can also execute the lib/hidapi/setup_task_helper.rb file directly. However, in your application, both of these may be too cumbersome. You can create an instance of the SetupTaskHelper class with the appropriate arguments and just run it yourself.

require "hidapi"
  0x04d8,             # vendor_id
  0xc002,             # product_id
  "pico-lcd-graphic", # simple_name
  0                   # interface

This will take the appropriate action on your OS to make the USB device available for use. On linux, it will also add convenient symlinks to the /dev filesystem. For instance, the above setup could give you something like /dev/hidapi/pico-lcd-graphic@1-4 that points to the correct USB device. The library can use these links to open the device, and the presence of the links in the/dev/hidapi directory would be a clear indicator that the device has been recognized and configured.


Bug reports and pull requests are welcome on GitHub at


Copyright (c) 2016 Beau Barker

As said before, this is a port of the HID API from Signal 11 so it has significant code in common with that library, although the very fact that it was ported means that there is no code that was copied from that library. That library can be licensed under the GPL, BSD, or a custom license very similar to the MIT license. This gem is not that library.

The gem is available as open source under the terms of the MIT License.


A ruby port of the HID API from Signal 11.




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