How to read data from HID device?

[anze87]

I am new at Raspberry and Python, so I will be very glad if you help me at my problem. I need to read some data from weight scale when button on scale is pressed. When I connected this scale via USB on PC, I saw that it works as HID. When I press the button, data are written as string in some text edit document for example.
So, now I am going to connect this scale on Raspberry Pi to read and manipulate with data. Can you give me some code example, advice, which library I need to import? I want to write a script, that waits for incomming data and then something do with it. Thanks for help.

[joan]

What happens when you plug it into a Pi and press a button?

[FLYFISH TECHNOLOGIES]

Hi,

data are written as string in some text edit document for example.

It seems that the weight scale represents itself as a standard HID keyboard.

In order to read its output, your software needs have focus and you read data 100% equally as when you type the values on your keyboard.


Best wishes, Ivan Zilic.

[anze87]

I finaly find time to moving forward. I try to read data, and it works just like from keyboard.
This works:

Code: Select all

buffer = input("new data")

I also try like this, but failed:

Code: Select all

f = open('/dev/hidraw0', 'rb')
buffer = f.read(8)

I don't know why incomming data are different in win8 and in Linux. For example, in Win I get string "G + 12.34 g" in Linux just "1234 ". When I read string data, I remove whitespaces with split() method, so I get just "1234". Because scale has fixed two decimal numbers, it's not a problem to divide with 100. But it becomes a problem, when result from weight is negative, and I can't know that. Does anybody know, what could be the reason for that? The result is the same, if I just open text editor and press button on scale, so I don't think it is problem in reading...

Thanks!

[Richard-TX]

Usually when a weight comes up negative, it means that it is either in overload, there was a weight on the scale when it was powered up (tare weight) , or there is an error. Try power cycling the scale with nothing on it and see if the negative values go away.

[anze87]

Yes, I understand that. But I can't figure it out why in Win is received data different than in my Raspberry. In Win I get sign (+) or (-) and decimal dot (.) and unit (g). In Raspberry I get just numbers and spaces after numbers, without anything. What would cause that, any ideas to check?

[FLYFISH TECHNOLOGIES]

Hi,

What would cause that, any ideas to check?

Weird... I first thought about NumLock,... but this doesn't explain behavior well...
Do you have a chance to sniff USB communication ?


Best wishes, Ivan Zilic.

[anze87]

Do you mean with some kind of software? Isn't USB very complex protocol?

[FLYFISH TECHNOLOGIES]

Hi,

Do you mean with some kind of software? Isn't USB very complex protocol?

There are also some software-only sniffers. Since I worked with hardware-based only so far, I cannot write much about these software tools... Anyway, check this: http://biot.com/blog/usb-sniffing-on-linux


Best wishes, Ivan Zilic.

[anze87]

It was very stupid problem. I had wrong keyboard emulation settings on scale. Now it works, as it should. Thanks for help!

[LeeAlexander]

I'm interested in using the Pi to read scales, could you tell me what scales you used?

Regards
Lee

[ghp]

Hello,
some time ago I connected a barcode scanner to raspberry, using my python scratchClient software and scratch.
I wanted to avoid the problem, that HID input is accepted only when the program runs in foreground.
http://heppg.de/ikg/wordpress/?p=390
Although the target here is to connect this to scratch, the python code is straightforward and you find it in scratchClient/src/adapter/usbAdapter.py

Regards,
Gerhard