Scale zero command needs to be a feature report

[skylarmt]

The scale zero command should work on any USB-compliant scale, but this code doesn't send it correctly.

I've had success in another codebase sending feature report 0x02 with a single byte 0x03. It took me quite a while to figure this out.

This should fix the error "errno=No such file or directory r=-1 (LIBUSB_ERROR_IO) transferred 0 bytes" when running ./usbscale zero

[erjiang]

Can you give me more info about the scale you are testing with, including the manufacturer and device IDs? I ask because the current implementation of zero does work for my scale, but apparently doesn't work for others.

[erjiang]

Can you try this branch #37 to see if it works for you? It does not work on my scale; I get:

zero command failed: errno=Resource temporarily unavailable r=-9 (LIBUSB_ERROR_PIPE)

[sanmai]

Several widely used scales do not even have the input endpoint, i.e. you can't send commands to them. At least I can see this app checks for write endpoints.

[skylarmt]

Well the change doesn't produce an error anymore (it outputs tared followed by the weight), but it also doesn't tare the scale. What you have looks like it should work, but it apparently doesn't. It is late here though so I might have missed something. For reference, I'm using a Mettler-Toledo PS15 for this test.

Last year or so I bothered Mettler-Toledo for a while and they eventually emailed me a giant PDF with their entire scale protocol specification (the USB HID POS spec plus vendor-specific additions), in a format that's a bit easier to understand than what USB-IF provides. Maybe it'll be helpful tracking down what exactly is wrong.

30126465 USB protocol BC scale family.pdf