Author Topic: Report lenght is different in USB Analyzer  (Read 1480 times)

moelski

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Report lenght is different in USB Analyzer
« on: June 11, 2009, 01:25:52 am »
Hi Roman,

today I started to communicate to another HID device. I don´t have a protocol description so I startet reading the Report length. The result was 24.
Then I used an USB analyzer to view the USB data. The USB analyzer told me that the report length is 23 bytes. So I was a little bit confused. I tried another USB analyzer and this reports 23 bytes report length, too.

I tried to write to the device and got no response. I kept my application running and started the original Device application. My application was able to capture the feedback from the device, too. It looks like this:
00 12 20 04 00 00 1E 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

As you can see there is a leading 00 at the beginning. Do you have any idea why the USB analyzer (keep in mind I used two of them) report without the leading 0?
I added the 0 to my test application and all worked fine.

So is there any USB specification that leading zeros could be hidden !?

Greetz Dominik

Roman Novgorodov

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Re: Report lenght is different in USB Analyzer
« Reply #1 on: June 11, 2009, 01:43:37 am »
Hello

The first byte is ReportID value. Possible USB Analyzer shows only actual length of data without byte reserved for ID of HID report.

Roman Novgorodov
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moelski

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Re: Report lenght is different in USB Analyzer
« Reply #2 on: June 11, 2009, 12:11:51 pm »
Hi Roman,

Quote
The first byte is ReportID value
You´r right. Thx for the information.