Today I’ve been playing with a commercial RFID reader and an Arduino UNO. I like the idea of this combo in principle, because I can connect a 12V power supply to the Arduino and power the reader directly from the VIN pin, eliminating the need for two power supplies or a step-down converter.
I tried several different libraries for Arduino, and wasn’t having the best of luck — I settled on Daniel Smith’s code from Pagemac back in 2012.
http://www.pagemac.com/projects/rfid/arduino_wiegand
Then I made a change that caused buffer overflows and Arduino resets. Once that was fixed, it started reading cards consistently. But it was reading them at twice the actual tag length. A 34-bit card was detected at 68 bits. I changed the pin mode from INPUT to INPUT_PULLUP on both data pins, and bang, I was getting 34-bit tag reads.
Unfortunately, the code I had only interpretation for 35 and 26-bit formats, so some minor rearrangement of boundaries and bitshifting was required. It’s easy to tell when bitshifting is required, because the result you get is a multiplier or a factor of the result you expect. In my case, the facility code was coming up at 1/8 of the value of the actual facility code, and the card code was coming up as 2x the actual card code (actual codes were validated by the Proxmark3 RDV4).
After the bitshifting was done, it was able to read my card properly. Now I just need to set up interpreters for all the known card variants that I need to test against.
BIG thanks to Kevin for his help in narrowing down the issues I was fighting with.