Data Acquisition

For full-duplex systems using balanced differential transmission, two sets of screenedtwisted-pair conductors are required in one cable, with both individual and overallscreens. The entire cable is covered with a protective PVC sheath.3.7.2 Coaxial cablesCoaxial cables are used in applications that require high data transfer rates of up to10 Mbps or high-frequency analog signals over long distances.Coaxial cables are more expensive than twisted-pair cables. They consist of a centralconductor running through an enclosing cylinder on the same axis. This enclosing cylinderis made of a conducting material and is braided for flexibility. The insulating materialseparating the two conductors affects the cable capacitance, and hence the rate of signalpropagation. The cable is usually covered with a protective PVC sheath, and sometimeswith an additional shield as well.Several types of standard coaxial cables are manufactured; each has a differentcombination of electrical and mechanical characteristics to suit different applications.The main variables are:• Cable characteristic impedance• DC resistance• Power capacity• Bandwidth• Type of shielding• Mechanical characteristics (such as flexibility)Coaxial cables are more difficult to terminate than multi-core or twisted-pair cables.They are also more difficult to splice and connect to tee-offs. Special tools andconnectors are required for good coaxial cable terminations. The ends of the cable shouldbe terminated in a dead-end terminator to prevent signals reflecting from the ends of thecable. Coaxial cables can sometimes be ordered to specified lengths and with terminatorsalready in place.