Introduction

The PCI/104-Express specification establishes a standard to use high speed PCI Express® bus in embedded applications. It was developed by the PC/104 Embedded Consortium and adopted by member vote in March 2008. The PC/104 Embedded Consortium chose PCI Express® because of its full PC market adoption, performance, scalability, and growing silicon availability worldwide. It provides a new high-performance physical interface while retaining software compatibility with existing PCI infrastructure.

Incorporating the PCI Express bus within the industry proven PC/104 architecture brings many advantages for embedded applications 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.

Background

The main objective in defining an addition of PCI Express to PC/104 was to preserve the attributes that have made PC/104 so successful in embedded applications, namely:

• Compact: 3.6 by 3.8 inches (90 x 96 mm) module size
• Self-stacking: expands without backplanes or card cages
• Rugged, reliable connectors: reliable in harsh environments
• Four-corner mounting holes: resistance to shock and vibration
• Fully PC compatible: reduced development costs and time-to-market

In addition, it was important that a stackable form of PCI Express take into consideration backward compatibility with current PC/104 Embedded Consortium specifications and form factors. The design had to support automatic detection of up or down stacking and had to have automatic link shifting to allow simplified, universal add-on module designs. The PCI/104-Express design approach provides a consistent and interchangeable path for the stackable PC architecture across the PC/104 Embedded Consortium’s 104, EPIC, and EBX form factors.

PCI/104-Express Specification Overview

Four x1 PCIe® Links

• One x16 PCIe Link (optionally configurable as two x8 Links, two x4 PCIe Links, or two SDVO Interfaces)
• Power: +3.3V, +5V, +12V, and ATX power and control signals +5V Standby, Power supply on, and Power OK
• SMBus

PCI-104 PCI Bus connector:

• PCI Bus: 32-bit, 33 MHz, Four Bus Master capable (same as on PC/104-Plus and & PCI-104)
• PCI Bus: 32-bit, 33 MHz, Four Bus Master capable (same as on PC/104-Plus and & PCI-104)

Here is a brief summary of the basic specs of PCI/104-Express modules:

• Number of PCI Express modules per stack: 4 x1 and 1 x16 (host may configure as two x4 or x8)
• Stack direction Stack up or stack down
• Data throughput PCI bus 132 M Bytes/sec (26 times ISA)
• Data throughput single x1 Link 500 M Bytes/sec (4 times PCI)
• Data throughput x16 Link 8,000 M Bytes/ sec (60 times PCI)
• Total data throughput all Links 10,000 M Bytes/sec (75 times PCI)
• PCI Express bus power +5V power capability 84 watts
• PCI Express bus power +12V power capability 100 watts

PCI/104-Express Connector

The PCI Express connector was specifically designed for the PC/104 Embedded Consortium to match the PC/104 standard 0.600 inch (15.24mm) stacking height and standoff tolerances. It was then tested to ensure it meets the PC/104 durability expectations and PCI Express signal integrity requirements. There will be multiple suppliers.

Link Shifting Stack Examples

Link shifting allows universal add-in card design and automatic PCI Express link assignment.

Up or Down Stack Configuration Examples

The flexibility and expandability of the bus and mechanical layout allow many different stack configurations to support an array of diverse project requirements. For example, the CPU can be positioned at the top or the bottom of a stack as shown in the figures below. See the full specification for more examples.

PC/104 Bus Evolution

PC/104 stackable embedded PCs have followed the desktop PC leveraging on the hardware and software support developed for this popular platform.

PCIe/104™

PCIe/104™ PCIe/104 is PCI/104-Express without the PCI bus. Since PCI Express is based on PCI technology, a PCI Express to PCI Bridge is straightforward. A comparison between a PCI/104-Express CPU and Stack and a PCIe/104 CPU and stack shows that both easily support PCI Express and PCI add-in cards.

Current Status of the PCI/104- Express Standard

Numerous companies have announced development and support for PCI/104- Express. Products demonstrated or under development include high performance single-board computers, 1Gbit Ethernet, high performance data acquisition and control interfaces, PCI bus adapters and bridges, and packet switches to provide expansion beyond six add-in cards.

Copies of the PCI/104-Express Specification are available to individuals and companies developing embedded systems. For further information, and for updates to this specification, visit the PC/104 Consortium site at www.pc104.org.

The consortium now has a completely redesigned and updated website. Visit www. pc104.org for the latest consortium and member company news. Be sure to visit the completely updated new products section to locate members’ products for your next project and to keep abreast of the latest PCI/104-Express product releases.

Where Does PCI/104-Express Fit?

The PCI/104-Express & PCIe/104 specifications continue to following the desktop PC and the path provided by the major chipset manufacturers.

• PC/104 supports ISA only
• PC/104-Plus supports PCI and ISA
• PCI-104 supports PCI only And now
• PCI/104-Express supports PCI Express and PCI
• PCIe/104 supports PCI Express only

Background on PC/104: the Need for an Embedded-PC Standard

Over the past decade, the PC architecture has become an accepted platform for far more than desktop applications. Dedicated and embedded applications for PCs are used as controllers within laboratory instruments, communications devices, and medical equipment, to name just a few examples.

By standardizing hardware and software around the broadly supported PC architecture, embedded system designers can substantially reduce development costs, risks, and time. This means faster time-tomarket and the ability to hit critical market windows with timely product introductions. Another important advantage of using the PC architecture is that its hardware and software standardized components are widely available. These components are also significantly more economical than traditional non-PC bus architectures such as STD, VME, and Multibus. This means lower product costs.

For these reasons, companies that embed microcomputers as controllers within their products seek ways to reap the benefits of using the PC architecture. However, the standard PC bus form-factor (12.4” x 4.8”) and its associated card cages and backplanes are too bulky (and expensive) for most embedded control applications.

The only practical way to embed the PC architecture in space and power sensitive applications has been to design a PC – chipby- chip – directly into the product. But this runs counter to the growing trend away from “reinventing the wheel.” Wherever possible, top management now encourages outsourcing of components and technologies, to reduce development costs and accelerate product design cycles.

A need therefore arose for a more compact implementation of the PC bus, satisfying the reduced space and power constraints of embedded control applications. Yet these goals had to be realized without sacrificing full hardware and software compatibility with the popular PC bus standard. This would allow the PC’s hardware, software, development tools, and system design knowledge to be fully leveraged. In other words, the PC bus had to be standardized while capturing all of the benefits the design engineer wanted from the PC bus.

PC/104 was developed in response to this need. It offers full architecture, hardware, and software compatibility with the PC bus, but in ultra-compact (3.6” x 3.8”) stackable modules. PC/104 is therefore ideally suited to the unique requirements of embedded control applications – including full standardization by an international group of over 160 companies who manufacture products based on our PC/104 standard.

Other PC/104 Specifications

The PC/104 Embedded Consortium maintains the PC/104, PC/104-Plus, and PCI-104 specifications on the 104 form factor as well as the specifications for the EPIC and EBX form factors.

Other PC/104 Specifications The PC/104 Embedded Consortium maintains the PC/104, PC/104-Plus, and PCI-104 specifications on the 104 form factor as well as the specifications for the EPIC and EBX form factors.

PC/104-Plus added PCI bus to classic PC/104 on the 104 form factor. 132M Bytes per second transfer rate made high speed processing possible in rugged embedded systems while the ISA bus allowed use of the extensive infrastructure of embedded modules.

PCI-104 actually existed in the PC/104-Plus specification, but it didn’t have a name. Instead of calling it “PC/104-Plus PCI only” forever, the consortium decided to give it its own specification and PCI-104 was born with only a PCI bus.

While not much new was happening in PC bus architecture, the consortium adopted two form factor specifications.

EPIC (Embedded Platform for Industrial Computing) was the first to be introduced. At 4.528 x 6.496 inches (115.00 x 165.00 mm), it is larger than the 104 form factor and allows room for tall cooling solutions for high end processors and space for standard PC I/O connectors.

EBX (Embedded Board, eXpandable) is the original five inch form factor of many single board computers. At 5.750 x 8.000 inches (146.05 x 203.20 mm) it has room for a complete computer with standard I/O and memory DIMMs but still features PC/104-Plus expansion for flexibility and expandability.

Trademarks The PC/104 logo, PC/104, PC/104-Plus, PCI- 104, PCI/104-Express, PCIe/104, 104, EPIC, EBX are trademarks of the PC/104 Embedded Consortium. PCI, PCI Express, and PCIe are trademarks of PCI-SIG.