150- W STP-A Connectors

W STP A Connectors

The original IBM wiring plan specified the Medium Interface Connector (MIC), drawn in Figure 9.2. This connector was used as the equipment connector on the IBM token-ring system and eventually standardized as IEC 807-8, but I don't recommend using it . The 150- W STP-A cable should, in my opinion, be used only for short jumper-cable applications, not for general in-building wiring. In jumper -cable applications the shielded DB-9 connector is more compact and more easily handled in the field.

Figure 9.2. IBM Medium Interface Connector (MIC), IEC 807-8.

One striking feature of the IBM MIC is that the two mating halves of the connection are identical, or hermaphroditic . There are no "male" and " female " parts. This idea was intended to reduce the number of parts installers had to carry in the field. There are two drawbacks to the idea, however. The first drawback is that the hermaphroditic design makes it look weird, so it is difficult at first to see how the two halves will mate together. If you are going to make something to be used by customers all over the world, it helps if they can see how to use it.

The second drawback is that the original connector came in a little plastic bag with nine parts, each of which had to be simultaneously held in place while crimping together the connector shell. The first time I saw the IBM MIC demonstrated, at an IBM facility in 1982, the technician dropped one of the parts and had to crawl around on the floor looking for it. [83] By way of comparison, UTP connectors come in one piece and are much easier to assemble.

[83] The IBM MIC was intended to eliminate the requirement for the two most ubiquitous tools of the data installation trade ”the screwdriver and the punch-down tool. During the demonstration I saw, instead of these standard tools the technician carried a pair of scissors to open the parts bag, wire cutters, small needle nose pliers (in case any of the parts were bent in the crimping process), and a whopping big pair of pliers with the ends wrapped in duct tape designed to apply just enough force to crimp the shell without cracking it.

The equipment-end connector used with FDDI and Ethernet 150- W STP-A installations is the shielded DB-9 (Figure 9.3). This connector was specified for use with FDDI (TP-PMD), Fast Ethernet (100BASE-TX), and for Gigabit Ethernet (1000BASE-CX).

Figure 9.3. Shielded DB-9 connector.

The shielded DB-9 connector goes by many names (see Table 9.1) and is available in many grades of electrical performance. The original specifications for this connector were not appropriate for high-frequency operation; however, shielded DB-9 connectors have been produced with electrical performance that extends well out into the gigahertz region (well beyond the ISO/EIA building-wiring guidelines).

Shielded DB-9 connectors designed for mounting on high-speed pcb substrates typically employ a straddle-mount configuration, which places the plane of the pcb directly between the two rows of pins. The straddle-mount configuration brings all the connector pins very close to the surface of the pcb (one row on top and one on the bottom). The close proximity of the pins to the pcb allows all the pins to launch signals directly into well-controlled microstrip layers on the pcb. This configuration eliminates the large exposed right-angle bends typically found in connectors with multiple rows of pins.

Table 9.1. Different Names for the Shielded DB-9 Connector

Beware when using straddle-mount connectors that they impose a mechanical constraint on the overall thickness of your board and also an electrical constraint having to do with the assumed separation within your board between the signal (outer) layers and the nearest solid reference plane layer. If your signal-to-reference spacing does not conform to the expectations of your connector vendor, then the pad width the vendor has provided for the signal connection will not be the correct width to maintain a proper circuit impedance as the signal passes through the connector. You will notice this effect at speeds above 1 Gb/s (see Section 5.4.4, "Tapered Transitions," and Section 5.4.5, "Straddle-Mount Connectors").

Tables showing the electrical performance of standard 150- W STP-A connectors appear in Section 8.4, "UTP connectors."

Table 9.2 lists the pin assignments used by FDDI and Ethernet for 150- W STP-A connections. This table provides two different pin assignment listings for devices with and without an internal crossover. As explained in Section 7.6, "Crossover Wiring," the customary LAN wiring arrangement presupposes a hub port equipped with internal crossover, and a client port without. In this case the wire is expected to connect the pins straight through, from pin 1 to pin 1, 2 to 2, et cetera.

Table 9.2. Shielded DB-9 Standard Contact Assignments

LAN ports equipped with an internal crossover pin assignment will always be marked with an X symbol.

If the connectors at both ends of a link are marked with an X or neither is marked, then an external crossover may be required.

The customary correspondence between connector pins and the wiring color code for North America also appears in Table 9.2. On a DB-9 connector, the wiring pairs are assigned to pin pairs [1, 6] and [5, 9].

POINT TO REMEMBER

• The equipment-end connector used with FDDI and Ethernet 150- W STP-A installations is the shielded DB-9.

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