DYMO Scale with WebUSB
This is a proof of concept to use "webUSB" to get weight data from a DYMO M25-US
Requirements
WebUsb requires some setup before it can be used.
- Enable experimental platform features chrome://flags/#enable-experimental-web-platform-features
- Must be on localhost or https://
Lessons
There are many hoops to jump through before you can start talking to the USB device in question.
- Open connection to the device with
device.open() - Set a default configuration.
device.claimInterface(interfaceNumber)- speak to the usb device / read from the device with
device.transferOutanddevice.transferIn(endpointNumber, maxBytesPerMessage)
This behaves like a file and will read the N number of bytes from the pointer. We solved getting the most recent data with recursing to the end of the buffer.
The DataView / Array buffer that passed into the promise of transferIn is highly dependent on your device. For the DYMO M25 we use an Uint8Array, the data is a 6 digit array where index 4 and 5 added together calculate the weight on the scale.
Uint8Array(6) [3, 2, 11, 255, 0, 0]
| Index | What is it? | Values |
|---|---|---|
| 0 | Scale Battery Level? | 3 |
| 1 | Scale State | Zeroed (2), Positive (4), Negative(5) |
| 2 | Unit Mode | Grams (2), Ounces (11) |
| 3 | Scale Factor | 0 or 255 |
| 4 | Raw Weight | 0-255 |
| 5 | Raw Weight Multiples of 256 | 0-255 |
Warning on Macs
In OS X USB devices that use the HID device class are opened by IOUSBHID and can not be used by in any meaningful way without writing a codeless kext or disabling IOUSBHID.
Disabling HID kext for development
sudo kextunload -b com.apple.driver.usb.IOUSBHostHIDDevice
It will disable any external USB keyboard.
sudo kextload -b com.apple.driver.usb.IOUSBHostHIDDevice
Will re-enable your external USB keyboard and disable the scale.
another option is to reboot your machine.
Massive thanks to http://steventsnyder.com/reading-a-dymo-usb-scale-using-python/ for a push to the right direction on how the data was being sent.