A Blackcat cable is the interface you'll need if
you plan on modifying a Motorola SB5100 below are
the schematics for making a Blackcat.
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And here's an alternative that doesn't require a
chip or resistors. There has been some talk of
frying your modem using this method but I've tried
this myself on a SB4100 and a SB5100 and it worked
fine on both modems.