The gang of five has popped the cork on the limited performance of the PC/104 architecture by incorporating PCI Express into the stacking architecture (Fig. 1). Ampro Computer, Octagon Systems, Micro/sys, VersaLogic, and WinSystems will be talking about this new technology at the Embedded Systems Conference in Boston (see "Embedded Systems Conference Plays Home to Boston Revolution," ED Online number 10947).
I introduced EPIC Express in "An EPIC Tale: PC/104 Hitches On To PCI Express" (ED Online number 10939), but there was not enough space to provide al the details of EPIC Express. This article works to correct that, including taking a closer look at the connectors used in the design (Fig. 2).
EPIC Express Details
Two components compose the EPIC Express board design: a baseboard and one or more stacked modules. The baseboard has the host processor and many of the standard peripherals found on a PC/104 single-board computer (SBC). Each EPIC Express SBC has the same form factor as an EPIC SBC, a PC/104 Plus-only solution. The same is true for the stacking modules. The difference is in the connectors and the plug-in sequence. With PC/104, the order of modules atop the SBC is arbitrary and does not affect the operation of the system.
This changes with EPIC Express due to its shifting nature (Fig. 3). There is some flexibility here, yet a strict order to follow. But first a few details about PCI Express in EPIC Express.
EPIC Express supports a PC/104 bus architecture plus four x1 PCI Express lanes (Table 1). It optionally supports two x4 PCI Express lanes. Any x4 PCI Express modules must be placed in the stack first, although the order of the two modules is arbitrary. Following these modules come one to four x1 EPIC Express modules. Finally, any PC/104 modules can be put on top of the stack in any order.
The number of modules in a stack is limited only by space and the restrictions just outlined for the PCI Express modules. As most PC/104 systems employ an average of only 1.5 modules, the EPIC Express architecture is expected to easily support current and future system requirements.
Connecting The Pins
Table 2 shows the list of pins found on the PCI Express connectors. The design approach allows an SBC to provide a subset starting with the Standard connector that has only the four x1 PCI Express links. This performance may be sufficient for many existing designs that employ the PCI-104 connector in a PC/104 Plus architecture. The EPIC Express design is faster and consumes less space, while providing more flexibility than a parallel PCI design. In addition, the hardware used, either PCI or PCI Express, is transparent to software operation. This is the same for PC-based PCI and PCI Express systems.
The Full connector actually spans the width of three Standard connectors and a Standard can be plugged into it. A Full connector will only be found on an x4 module, while a Standard connector will only be found on an x1 module.
The connectors are the key to the success of EPIC Express. They are available from Samtec, the company responsible for designing and testing the connectors. EPIC Express does not work without these connectors. They provide the quality necessary for high-speed PCI Express serial communication.
In addition to the shifting logical connections, the neat thing about Samtec's connector design is that it does not require any active components. This reduces cost and complexity.
Waiting For The Train
Although not on the shelf yet, EPIC Express modules are expected soon. The design moves PC/104 into new areas that it could not originally compete in. Although investigations are already looking into an x16 video connection, the existing design is more than suitable for high-speed applications, such as Gigabit Ethernet and video processing.
EPIC Express also opens up the possibility for other features such as support for ExpressCard (see "ExpressCard Replaces PCMCIA," ED Online number 10183). PCI Express also opens significant advances in chassis extensions, as well as the potential for hot swap services that were unavailable with PC/104 and EPIC.
It will take some time for EPIC Express to reach critical mass. But it will now come sooner than later given this new architecture and the plethora of PCI Express chips and devices becoming available through other channels.