Ok, there are only a few radios I'm aware of that can go direct from PC Serial port to unit, and the only one I can guarentee is a MTR2000 repeater. Got a minute for the rest? Other vendors, brands do it...but not many Moto ones.
The motorola RIB (Radio Interface Box) is an interface box that allows modern (sic) computers to speak the logic levels and speed that most older motorola radios use for data transfer. The older the radio, as long as it doesn't require eproms, the higher the need for a motorola compatible RIB goes. The newer radios (that were in development during the active lifecycle of windows XP IIRC) are the ones that are more tolerant of USB interfaces. I believe the newest product lines Moto is carrying is completely USB dependent, and do not have any RIB compatibility.
The motorola RIB (or a polaris box, or a Sandy-rib) are required to connect the computer's RS232 levels and data from the com port, to the individual radios. You cannot skip this step.
If the radio you are trying to program did not have (at least at some point) a motorola produced / designed USB Rib-less cable somewhere in its product cycle, you need a RIB. If there was a USB option for programming cables produced by moto for your product, you are in luck, you are not completely dependent on the RIB.
There's another site out there
Batwing Laboratories which is a veritable cornucopia of information about this, i'd suggest browsing it...i'm giving you a 10 second primer.
Depending on what radios you are trying to program, the older the radio, the more RIB dependent you will be.
I'm not even going to delve into the computer processor requirements that come with older vintage motorola radios, many of us have 286 & 386 laptops and desktops being kept running with hopes and dreams, as modern computers are just too damn fast for some of the older RSS packages produced many years ago.
(sigh) this is a huge topic. You need to research it extensively if you are going to get into programming some of the older radios, the nuances are mindblowing.
What kind of radios are you looking at in particular? Are you talking about the MTS2k and Astro's listed in your sig?
(there are some absolute Moto programming / service deities amongst us on this board, who have forgotten more than I can possibly ever have known - they're an invaluable resource, and can probably steer you right)