Step 1. Use a sharp knife to strip off the insulation from about 2/3 of the length of the cable. Seperate the wires into two strands, 2 and 4 wires each. Use heat-shrink insulator material to insulator the two strands again, so in the end your cable represents an Y-Fork.
Step 2. Solder the 2 wires of the first strand to pins 1 (+5V) and 4 (GND) of the 15 pin plug.
Step 3. Solder the 4 wires of the second strand to the appropriate pins of the 25 pin plug.
Step 4. Solder the 6 wires on the other end of the cable to the 7-pin mini-DIN connector. Solder the two GND wires from the different plugs together, and add short lengths of wire to connect pins 1-7 and pins 2-5.
|mini-DIN plug||15 pin D-SUB plug|| 25 pin D-SUB plug|
|1,7, GND||4, GND||18-25, GND|
|2,3, +5 V|| 1, +5 V |
|4, Read|| 10, Ack|
|5, Write|| 3, Data 1|
|6, Sense|| 12, Paper End|