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https://supportcommunity.zebra.com/s/article/Resetting-the-RFD8500-RFID-Sled?language=en_US
Update firmware:
Use the 123Scan Configuration tool from zebra.
To connect the sled I had to remove the current version and install the new 64bit version. When it is connected the green light in the back is on.
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https://www.zebra.com/us/en/support-downloads/rfid/rfid-handhelds/rfd-8500.html
Setup Region:
To use the RFID Sled the region needs to be set. This can be done in the configuration tool 123 Scan.
NB. The region must be set up for the RFID to work.