The brand or shape of the reader doesn’t really mean anything. You should get the DT Diagnostic card (https://dangerousthings.com/shop/rdc/) to see if your xEM might be compatible. If it’s HID and it is a low frequency reader, you will most likely need to put the xEM in HID mode.
If you are going to get the card, you should get the 125khz xLED as well so you can visualize the best way to couple your implant to a 125khz reader.
It is possible that the readers you tried are high frequency readers and not compatible with your implant.
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