A shielded cable usually has its signal wires surrounded by tin foil or aluminum foil inside the outer jacket. A bare wire is also wrapped around the foil to make contact with the foil shield. This bare wire is the shield connection. If the cable goes from point A to point B, your signal wires inside the foil will have connections at both ends. However, the bare wire should be connected to ground only at one end, preferably the end away from the noise generating device (motor). If your shielded cable goes from the motor to the computer, ground the bare wire near the computer end and leave the bare wire at the motor end unconnected. Of course you should still have another separate ground wire connecting the motor ground to the computer ground, or connecting all grounds together. Without the shielded cable, the motor could induce noise into the signal wires. If the shielded cable were connected at both ends, the motor could still induce noise into the signal wires through the bare wire that runs along the enitre length of the shielded cable, which would render your shield useless. If the bare wire at the motor end is unconnected, the shield is at ground potential with respect to the computer, and effectively shields the inner signal wires from EMF noise.
Don't get a shielded signal cable confused with a coax cable. They are two separate things. Coax has only one center conductor, shielded wire comes in a variety of numbers of wires inside the shield. Coax is meant for RF and it has a characteristic impedence (like 50 ohms for RF transmission and 75 ohms for TV signals). Coax is usually terminated with a special connector, like BNC or Type N or SMA, etc. Shielded cable wires can be stripped and connected with crimped connectors on each wire or even soldered into place. Don't use coax for non RF signals because the impedence could upset any AC signals running through the wire (depends on the impedence of the generating and receiving devices). Another useful wire is twisted pair. Two wires are twisted along the entire length. You can have a number of twisted pairs inside one jacket. This is usually used in telephone applications, and is very good for differential signals. By being twisted, any EMF induction is cancelled by the orientation of the two wires. The induced noise in one wire is 180 degrees out of phase with the induced noise in the other wire which causes cancellation. You can even get shielded twisted pair wire which is the ultimate to use with differential signals and would probably be your best bet to use. Look into this type of wire and use it if affordable. This will go a long way to protect your signals from the motor EMF noise (again, ground the end away from the motor).
Whew, this is getting too long...
