Electrical Engineering Terminology
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Twisted Pair Cable

Twisted Pair. A line consisting of two insulated wires twisted together to form a flexible line without the use of spacers. Different types of cables will vary the wire gauge [AWG] for the inner conductors and their twist, the number of times the wires are twisted per inch. The Braid or shield [outer conductor in the diagram] also varies with different cables by coverage and number of braids. The shield may also be a foil instead of a braid.

shielded twisted-pair cable diagram

Shielded Twisted Pair Cable

Electrical Characteristics to account for; Characteristic Impedance, Attenuation, and Capacitance.





Twisted pair cables may be shielded or un-shielded. The diagram above shows an example of a shielded twisted pair cable [STP]. Twisted pair cables with out a shield are called Unshielded Twisted Pairs [UTP], shown to the right. By definition a twisted pair cable is a multi-conductor cable, and normally a round cable. Also called a Paired Cable.

Ribbon Cable is also available as Twisted Pair, as a number of Twisted Pairs glued together into a flat cable [normally un-Shielded]. Normally a Twisted Pair Ribbon Cable will have a flat [un-twisted] section every three or four inches for IDC Termination [which may be a few inches wide].

The wire pairs of a ribbon cable can be run one of two ways; balanced or unbalanced. With unbalanced wire pairs the wire twist consists of a signal wire and ground. With balanced wire pairs the twist consists of a signal wire twisted with its differential pair [compliment]. If possible separate both the balanced or unbalanced wire combinations by a signal ground. Also try not to place signal wires on the cable edges, which should be ground wires.

Another consideration is wire termination on the connector. Try to terminate cable pairs on the same row of the connector, so the wire pairs are adjacent to each other. The connector pin length [lead length] from one row to another will be different, as one row will be closer to the Printed Wiring Board than the row above it. When the pair is on the same row the pin length is the same, while row to row lengths will result in different wire runs causing signal skew between the wire pairs.

Also refer to Twisted pair Attenuation Graph for 24 AWG wire.

Military Standards: MIL-C-17

Interface Bus Standards: EIA-485 is one of many electrical interface buses that required Twisted Pair cables.
CanBus may use either a STP or an UTP cable depending on the application [the level of noise in the environment].

A table that provides a number of Cable Bus Standards using Twisted Pair Cable and which use STP or UTP cable interfaces.

COST: There is the added cost of the shield for Shielded Twisted Pair Cable, so Un-Shielded Twisted Pair Cable is cheaper to implement. Systems that require higher reliability or that reside in high noise environments will tend toward the higher cost Shielded Twisted Pair Cabling.
A shielded cable may have a shield surrounding each pair or an over-all shield covering all the twisted pairs within the cable. A cable may also be double-shielded. Each different configuration of shielding having a different cost associated with it.

Unshielded twisted-pair cable
UTP Wire

Refer here for Manufacturers of Wire and Cable
Additional design hints; Cable Assembly Guidelines

With any cable design there may be a requirement to run spare wires, don't leave any unused wires in the cable floating. Unused wires within the cable should be grounded. If the wires are for future use tie the wires to a ground location that is accessible.

 
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