You should look very closely at the interface definition for the magnetic card reader. Very often these supply a clocking signal that accompanies the data and will give a wave form that looks similar to the following:
If your interface has both of these signals then you need to connect both to port pins. The process for recovering the data inside of the 8052 will be to run a set of nested loops. The outside loop will be one that counts the clocks/bits coming in. The inner loop would poll the CLK port pin until the rising edge is seen (ie a change from low to high). Then immediately following the change the bit from DATA pin would be sampled. A very typical way to sample the data is to move the port bit to the CY flag and then rotate the CY flag into the ACC register.

Michael Karas