Robot identification

I am trying to find out a way to have a inexpensive way to identify multiple robots when they move over a small area. I was going to use RFID but that is to much... Would infrared work? All I need is a system that can read a 3 digit ID from each robot and then relay the ID to a computer, any ideas?

Haha sorry I didn't do a very good job of explaining myself. A robot will be driven into a "slot" and the transmitter will be on the bottom of the robot, the reader being under the robot. Only one robot will be in each slot at a time, but there will be multiple slots. This is the only point that the data will be sent.

Would this setup work? http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1211371364

robotix3: Haha sorry I didn't do a very good job of explaining myself. A robot will be driven into a "slot" and the transmitter will be on the bottom of the robot, the reader being under the robot. Only one robot will be in each slot at a time, but there will be multiple slots. This is the only point that the data will be sent.

In that case, IR would probably be the simplest to implement - but another possibility might be an air-gap transformer (1:1 ratio) system, with half of the "transformer" on the robot, and half under the slot. It's something to think about, but an IR system would likely be easier and cheaper (not too mention easier to align and "tune").

Just realized, will I be able to use multiple detectors with one arduino? Would it be easier to have 2 contacts on the robots that can send some kind of TTL? Could someone give me an example? :D

After doing more designing I decided that contact would be best... 4 connections at least per arduino would be ideal.

And speed would be in a margin of a few seconds

I guess I can just use the software serial interface to use multiple digital pins on the "hub" arduino for RX and ground. Hmm would this work?