Way to determine order of cards plugged into controller board
If you could add another line with high resistance per length you could connect one end to positive power supply and other to negative power supply in this case each card can measure its position and report it to master. 

Other possibility would be that at master that line is connected to positive power supply with known resistance and when card is inserted it would connect negative power supply at the line at its position, master can then measure position itself and then tell card to disconnect from that line.