PCI/104-Express simplifies peripheral board circuitry
Incorporating the PCI Express bus within the industry-proven PC/104 architecture provides embedded applications with many advantages, including fast data transfer, low cost due to PC/104’s unique self-stacking bus, high reliability due to PC/104’s inherent ruggedness, and long-term sustainability. The PC/104 Consortium’s specification design goals aim to make implementation easy for peripheral board manufacturers and users. Connector selection, link shifting, stack-up or -down, and universal peripheral circuitry embody these goals.
The PC/104 Consortium specifically chose a single connector for PCI/104-Express and designed the specification to make all peripheral boards universal. Designers only have to make one version of a peripheral board regardless of its position in the system or its use of any bus signals. Although a single three-bank connector might cost more than a smaller connector, using two one-bank connectors of the same type is actually more expensive than using this three-bank connector.
Some PCI Express specifications have multiple connector versions and allow systems to be built in any combination. While this sounds very I/O-centric, let’s consider a simple case involving only two connectors, Type A and Type B. Manufacturers can build host CPU boards with either Type A, Type B, or both connectors – three options requiring three different PCBs. The question is: Should peripheral card manufacturers build boards with Type A, Type B, or both connectors? If it is a stacking system and they use Type A, then no Type B boards can be used in the system. If they pick Type B, then no Type A boards can be used. In the end, manufacturers might decide that the only way to be universal is for all their boards to have both connectors. Let’s hope they’re not using the same connector family as PCI/104-Express, or they’ll find the cost greater than a single three-bank connector.
The connector is the key component to a great specification. It is no accident that the PCI/104-Express connector consists of two surface-mount components. A surface-mount connector allows the top connector to have different signals compared to the bottom connector. This is critical for PCI Express and USB link shifting in a stackable architecture. To get an idea of how this operates, imagine a host CPU with four PCI Express x1 links. Each x1 link can be connected to a single peripheral device. If the links are numbered 1, 2, 3, and 4, then peripheral board 1 must use link 1, peripheral board 2 must use link 2, and so on. But a truly universal peripheral board would always use the same link.
PCI/104-Express peripheral cards always use the same link. If the peripheral card is placed above the host CPU, it will use PCI Express link 1 from its bottom connector. It will then shift link 2 on the bottom connector to link 1 on the top connector, shift link 3 on the bottom connector to link 2 on the top connector, and shift link 4 on the bottom connector to link 3 on the top connector. Another peripheral board can be placed on top of the first and can use link 1, which is the CPU’s link 2 shifted into the link 1 position by the first board. Notice this link shifting is automatic for the user. There are no slot switches or address jumpers to set. Simply stack the boards in any order and they work.
Another feature of PCI/104-Express is the CPU direction (CPU_DIR) signal on the bus that indicates to the peripheral board if it is above or below the CPU. Taking advantage of the double surface-mount construction of the connectors, the CPU ties its top connector CPU_DIR pin to ground and its bottom connector CPU_DIR pin to +5 V. A peripheral board above the CPU will see a ground on the pin, while one below the CPU will see +5 V on the pin. This signal requires no user settings, making stack-up or -down truly plug-and-play. CPU_DIR is important because boards below the CPU will always use link 4 from its top connector, but as illustrated in Figure 1, the shifting is always the same.
PCI/104-Express meets the PC/104 Consortium’s goals of simplifying implementation for peripheral board manufacturers and users. A single three-bank connector means that one peripheral PCB will work in all PCI/104-Express systems. Automatic link shifting and stack-up or -down provide a truly universal peripheral board that can be stacked in any location without worrying about jumpers or switches. PCI/104-Express is another winner from the leader in stackable PC specifications.
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PC/104 Consortium 916-270-2016 email@example.com www.pc104.org