Make sure that your cable has all the wires connected to all the pins. (There are DB25 serial cables that look identical, except, they only have a few of the pins wired - DO NOT USE THESE)
If you want to be absolutely sure, get a ohmmeter and check, pin by pin, that pin1 on one end matches pin1 on the other end. Repeat this for all pins and ensure there are no shorts between the pins.
Parallel Port cables and SCSI port cables have been known to work. You can make your own if you are patient.
The requirements are:
You can use flat ribbon cable and crimp-on "IDC" connectors.
The original Apple cable is round with a silver/grey colour, and part number 590-0202-B moulded into the connector hoods.[LisaFAQ] > [Hardware] > [Hard Drives] (Comment on this answer)