here is no schematic that I've found for the Mach3 CNC-machine interface board, but the signals are connected as follows, according to silkscreen text on the board:

^ DB25 connector pin      ^ PC Parallel Printer Port Signal      ^ CNC control function         ^
| 1                       |    C0                     |   unused     |
| 2                       |    D0                     |   X step     |
| 3                       |    D1                     |   X Direction  |
| 4                       |    D2                     |   Y Step       |
| 5                       |    D3                     |   Y Direction  |
| 6                       |    D4                     |   Z Step      |
| 7                       |    D5                     |   X Direction  (unused)    |
| 8                       |    D6                     |   A Step      (unused)  |
| 9                       |    D7                     |   A Direction   (unused)    |
| 10                      |    S6                     |   E-stop     |
| 11                      |    S7                     |   unused?      |
| 12                      |    S5                     |   (X axis limit)      |
| 13                      |    S4                     |   (Y axis limit)     |
| 14                      |    C1                     |   Common motion enable  |
| 15                      |    S3                     |   (Z axis limit)      |
| 16                      |    C2                     |   B Step      |
| 17                      |    C3                     |   B direction or Spindle Enable      |


The interface board must be cabled to a parallel printer port on a PC.  The parallel port must either be on the motherboard or on a PCI expansion card; USB control is not possible (see the linuxCNC documentation).

If curious, more parallel port information is at https://en.wikipedia.org/wiki/Parallel_port