frandiox / rfg-scale

A DYMO scale component using WebUSB for Ruby For Good.

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.

1. Enable experimental platform features chrome://flags/#enable-experimental-web-platform-features
2. 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.

1. Open connection to the device with device.open()
2. Set a default configuration.
3. device.claimInterface(interfaceNumber)
4. speak to the usb device / read from the device with device.transferOut and device.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.