PDF of this tutorialAll telecommunications equipment must be physically connected to the network at some point. In an E1 or T1 switch, this means that every single E1 or T1 PCM trunk interface must plug into the appropriate termination point on the switch system. As PCM density grows, so does the tangle of cables. Clearly, to maintain operational effectiveness, a simple means of combining high levels of connectivity with ease of access to the card must be found. This is particularly important when one considers the operational implications of truly hot-swappable systems. The work of the PICMG has led to an elegant solution to this problem.
As indicated earlier, it is possible to connect a second board into the reverse of the vertically orientated CompactPCI bus interface. This permits manufacturers to design boards that serve only to terminate external input and output interfaces. All processor activity may be concentrated on the front-panel portion of the card, allowing all cabling associated with a particular card to be plugged into an electrical interface at the rear of the system. Because it is divided into two sections, the front or processor section—when it must be replaced—can simply be removed using the physical ejector levers provided without disturbing the cabling secured to the rear portion. Technically, the rear card is known as a break-out or transition board.
The transition board is connected to the front-panel card via a mirror image of the connection interface utilized by the front card. All of the input and output interfaces for a particular rear or front combination are presented through the J5 connector pair of each, which passes across the CompactPCI backplane (see Figure 3).
Figure 3. Transition Board
The elegance of this system is underlined by the fact that, once again, interoperability is assured, for the IEEE 1101.11 specification defines the characteristics of the rear input and output transition card and indicates that this card should connect to that of the front panel. The transition card has a maximum depth of 80 mm, which ensures that the depth of a CompactPCI system will not exceed 300 mm. CompactPCI systems may thus be used in space-sensitive environments, particularly as they retain the small footprint that made 19-inch, rack-mounted PC systems so popular.
