We bought some Magnetic stripe cards and I'm writing a program that will get talked to by a mag card reader. What I need help with is that I can't, for the life of me, find out what characters are valid on track 1,2, and 3. I have my program working, but I would like to have the program raise red flags if someone tries sending it data that wouldnt be able to come from a mag stripe. 

Question W/O background: What characters can be encoded on T1,T2,T3 of a mag stripe card? Sources please!