Barcode Scanner Decoding

Question

I am experience some trouble decoding the output of a 1D Chinese Barcode Reader. The reader uses a USB interface and connects as a Keyboard HID device (which I have no problem with). After interfacing the device with Labview and generating the inf driver file I tried reading device interrupt data from a test barcode in the configuration manual "000200" the output of the Device is sent serially and is as follows "39 39 39 31 39 39 40".

I am guessing that 40 is the escape character the 39 is 0 and the 31 is 2. After doing some research I could not find the relevant key code table for this encoding. I have tried disabling all other encoding formats using the configuration manual (39, full ascii, int 2 to 5..).

The module was able to read Upper case letter and send an additional character noting that it is an Upper Case

The device stopped reading the barcode after disabling the Code 128. I re-enabled this option and reading was successful. however the code 128 table have the "G" assigned to the 39 output and not the 0 which messes up the reading.

Did anyone work with the following format? if so which key code is it? or should I map the character set manually?

The following is a link to the purchased Module: Reader

Thank you it is much appreciated!

Answer 1

As per this answer, a USB HID device sends USB usage codes, not ASCII character codes. That answer links to the lengthy official documentation on usb.org, but this document from microsoft.com appears to be a concise summary. If those links break in future, a web search for usb hid key codes or similar should find an equivalent.

Looking at the HID Usage ID column on the Microsoft document, the code for '0' is 27 in hexadecimal, which is 39 in decimal. '2' is 1F which is 31, and 40 decimal is 28 hex which corresponds to Return. That would be consistent with the output you're seeing, assuming you're reporting it as a sequence of decimal values. As you've observed, a capital letter is sent as two codes, the first of which will probably correspond to the 'shift' key in the HID usage table.

You could try searching or asking around for a LabVIEW VI to translate these codes into ASCII characters but it's probably quicker to build your own based on the table linked above. To test it, you could use a barcode generator program or webpage to create barcodes for all the characters you want to be able to decode and check that scanning them with your device gives the correct output.