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I'm using Raspberry Pi Model B+. I'm new to the RPi and I don't really know the search terms for my question.

I have four RFID readers connected to the four USB ports of my RPi. I wanted to know if there's a command in python to know which out of the four readers/USB ports the data is coming in. The RFID reader prints hex or oct number with or without carriage return upon placing an RFID card.

The closest I get is alphacharlie's answer here (Capturing serial number of 2 USB RFID Reader in python (PI+2 RFID (Mifire RFID))) talking about /dev/input/keyboard. He/she said,

If one of your readers is of the HID type, then you can look for the input at some variation of /dev/input/keyboard

I think my reader is of the HID type, but I don't quite understand what "you can look for the input at some variation of /dev/input/keyboard" means.

Edit: I'm using USB MIFARE™ Reader SL040A (http://www.stronglink-rfid.com/en/rfid-readers/sl040a.html)

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I'm looking at the same problem with (currently) a single but different RFID USB (HIDevice) reader. It presents a 10-digit number and a (CR +?) LF (from the tag read) as if it was typed in at a keyboard. I made some headway with adding a new rule 10-local.rules to the /lib/udev/rules.d/ system to add a known name as a symlink in the /dev directory:

ACTION!="add|change", GOTO="end"

# Adding a standard symbolic link for RFID scanner, ideally will use to isolate from keyboard
SUBSYSTEM=="usb", ATTRS{idVendor}=="vvvv", ATTRS{idProduct}=="pppp", ATTRS{serial}=="############", SYMLINK+="rfid#", TAG+="rfid", OPTIONS+=last_rule

LABEL="end"

In your case you will need to edit vvvv and pppp to be the vendor and product id of the readers and then copy this three more times and replace ############ with the individual serial numbers of your devices (I assume they all all the same device!) and the # in rfid# with I suggest a number from 1 to 4 or (perhaps a "_location" for each device). Now, when each RFID USB device is plugged in it is detected and the rfid# symlink will appear and become available as, say /dev/rfid1 to /dev/rfid4 respectively.

The complication is that the input will still get treated as if it was typed at a keyboard - which will be treated as username entry on the local console as if there is a user trying to log in locally on the Pi. Further work is needed - possibly to get the device(s) to be ignored as keyboards and have a daemon to open those devices and monitor the input...

A (stupidly insecure) use of an RFID device (as a keyboard) as an access device:

As I happen to know the numbers produced by the tags I have and I only have one reader, I was able to create users with the same usernames as those numbers (sudo adduser can be a bit iffy about this, I think I had to use the --force-badname option). By disabling a password for that "user" and replacing the login shell by a shell script I could get that script to be run every time the relevant tag was presented to the reader - excerpt from in /etc/passwd:

...
1234567890:x:110:65534::/home/1234567890:/usr/local/bin/door_unlock
...

1234567890 is NOT the number of one of my tags...!

Whilst this works it is not secure - anyone getting even remote access to the Pi could try a login (via any enabled telnet/rlogin/ssh system) with the given number and they would succeed (no password remember!) and cause that door_unlock script to run.

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