Arduino compatible RFID reader, that detects when objects leaves detection zone

Hello, community. Currently i’m trying to find RFID reader, that allow me scan a tag, get data from it and detect, when it’s leaves detection zone. As i understanding, some of the readers on the market are high-level and sending the data only when detection phase was passed.

Most REFI readers continuously generate code while a token is in the field. However sometimes this is reduced to a one shot before it is output. Some have an signal that just detects the presence of a token.
However your words:-

that allow me scan nearest tag,

makes me wonder if you know how RFID works. Most will only read one token in the field at one time and two tokens in the field will stop either being read. Collision detection is possible but these tokens are more expensive.

Thank you for your reply. Yes, i know, that only one tag can be scanned usually - this was my misspell, fixed it. Can you please recommend me the model of reader and the model of tags, that allow collision detection? Also, do you know how many RFID readers i can connect to one arduino uno?

Can you please recommend me the model of reader and the model of tags, that allow collision detection?

Look at this thread:- http://forum.sparkfun.com/viewtopic.php?t=19307

Also, do you know how many RFID readers i can connect to one arduino uno?

Well I am building something with 32 RFID readers at the moment.

Thank you for your help. If i understanding right, the basic RFID reader using two digital pins for data transfer, so can’t understand how you connecting 32 of it to Arduino. Can you please describe the process and name the model of reader?

If i understanding right, the basic RFID reader using two digital pins for data transfer,

You can get them to just have one serial line.

Can you please describe the process and name the model of reader?

I can't because I designed it myself. Basically it uses data selectors to determine what to read.