Arduino, rfid card reader and long wire

Hi all,

First of all, sorry about my english. I'm doing my first experiments with Arduino and I have an issue that may be someone can help me.
I made an access control for office using an Arduino and RFID card reader (and an electronic lock to the door). Everything worked perfectly before installing it, but when installing it has stopped working. That's when I realized about problem: between the card reader and arduino is a 7 meters wire. Can anybody give me a clue about how can i solve this problem?

Thanks in advance!

RS485 might help a lot:

You don't necessarily need the software I described, simply putting the transceiver chip (at each end) could be enough.

Thanks you so much for the answer. I will try this option.

Thanks again!