Conclusions

This code showed how the reserved lines on the smart card are used in conjunction with credit increments and decrements. This is an analysis of how it triggers a credit deduct or add on the card:

    DEDUCT $0.10:

          ___________             ___________
_________|           |___________|           |__________________ Reset
                      ____________________________________
_____________________|                                    |_____ Clk
          ___________
_________|           |__________________________________________ I/O
          ___________
_________|           |__________________________________________ Rsv1

    Then issue write command:
00011001 00101000 11111111 00111100
01001001 00000000 01100010 10001101
11111111 11111111 01110111 10101101


    ADD $0.20:

          ___________             ___________              _____
_________|           |___________|           |____________|      Reset
                      ____________________________________
_____________________|                                    |_____ Clk
_____________________________________________
                                             |__________________ I/O
          ___________________________________
_________|                                   |__________________ Rsv1

    Then issue write command:
00011001 00101000 11111111 00111100
01001001 00000000 01100010 10001101
11111111 11111111 01110111 10101101
                                                           _____
__________________________________________________________|      Reset
 ________             ___________             ____________
|        |___________|           |___________|            |_____ Clk
 ____________________             ________________________
|                    |___________|                        |_____ I/O
 ____________________             ________________________
|     1 Credit       |___________|        2 Credits       |_____ Rsv1

Since the parking meter will refund whatever remaining amount there is to the card and doesn't have to do it one at a time like with decrements, the write command supports writing multiple credits back onto the card. Simply repeat the waveform above and assert Reset when you're finished "refunding" however many credits you want.