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4.1.5 Flat-Ribbon Cable
A key electromechanical feature of AS-Interface is the flat-ribbon cable. This cable design
is used because the AS-i cable should be contacted where possible without cutting off or
stripping the sheath. In order to avoid the risk of polarity reversal of the wires, the flat-ribbon
cable has a keyway at one end. Unless forced mechanically, the cable cannot be inserted
incorrectly in the cable routing aids, which prevents unintentional polarity reversal. The
trapezoidal cross-section of the cable makes it easier to seal the cable in the devices, thus
providing IP67 protection.
In addition to the mechanical keying, both wires in the flat-ribbon cable are color-coded: the
positive cable has brown sheathing and the negative cable has light blue sheathing. The
external sheath is usually yellow, and when the same cable type is used for the external
auxiliary voltage of output devices with 24 V DC a black sheath is used.
Figure 4-8
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AS-i flat-ribbon cable
The cable sheath can consist of various different types of insulation material. For normal
rubber cable, the sheath ensures that it automatically closes any contact points that have
opened again. This effect is known as self-healing. This means that multiple contact and
release is possible at the same point without adversely affecting the seal and the cable
insulation.
Other cable materials are also available, which have a high level of resistance to chemicals
and can permanently withstand UV light. However, the selected material does affect the
mechanical properties of the outer cable sheath.
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