Don't Let a Cable Clamp Damage Your Interface Ribbon Cable

[Tommy Tyler]

I've noticed that the 2-piece metal cable clamp supplied with most DB connector hoods is often used with flat ribbon cable, even though it is designed to hold round cables of 1/4" diameter or larger. I recommend you take care not to use it in a way that might cause failure of the cable over time. The problem is the very thin insulation (about 0.010", or the thickness of three sheets of paper) on the ribbon cable wires. If the clamp is holding the cable tightly, those ribs or sharp edges of the metal can gradually squeeze through the soft vinyl, and eventually short out a couple of wires. If your interface suddenly stops working for this reason, you'll go nuts trying to figure out why.

Sometimes, with large (DB25) hoods, the clamp is used by folding the ribbon cable back and forth until that there are five layers in the clamp. That's better, because the additional layers provide a sort of "cushion" that reduces the pressure on the insulation. However, it would be even better if the folded cable were wrapped with electrical tape.

The two techniques that concern me most are inverting one of the clamp pieces so it nests in the other, and flattening the clamp pieces slightly before using them. Both these methods put the cable in a metal "vice" that places an uncontrolled amount of pressure on the insulation. Here again, the problem can be eliminated by wrapping the cable with lots of electrical tape.

Personally, I prefer to avoid the metal clamp whenever possible by using a short piece of rubber tubing and a nylon tie wrap. For the tiny DB9 hood, which has a cable exit hole of 9/32" diameter, lay the cable along a piece of 1/4" O.D. x 1/8" I.D. tubing about 1/2" long. Fasten the tie wrap near the inner end end and let the other end protrude out of the hood. For the larger DB25 hood, you'll need rubber tubing that's 7/16" O.D. Various sizes of tubing can usually be found at Home Depot or other hardware stores.

In summary, if you use the metal cable clamp, PLEASE wrap the ribbon cable with vinyl electrical tape to protect it.

Tommy