Ribless cables basically have a rib built into the cable (as the other poster said)
the only problem is, there have been some ribless cables out there that were of questionable quality as the components they use for the protocol conversion are not exactly the same as the Motorola orig.
Also, the power it gets is from the computer - so this is a variable, I have one machine here that ribless cables don't work on - the power from the port isn't stable enough.
There are many variables in the "ribless" world, and the bottom line is a REAL rib is hard to beat because they work so well.
Many newer radios are only available with ribless style cables (XTS 2500/5000)
I find the newer radios are less picky than the other ones when it comes to voltages and timings so you will have better luck.