Need performance? Pick your Type
In 2012, the 20th anniversary of the PC/104 Consortium, you would think that the decision of which PC/104 product to use in your next project would be an easy shot, but it’s not that simple. For those who have been in the industry for some time, the word “PC/104” does not represent a single product type, but a range of product types that each have their advantages.
New and improved
The older PC/104 formats include PC/104 – ISA bus only; PC/104-Plus – ISA and PCI bus interface; and PCI-104 – PCI bus only. These formats are excellent for lower- and mid-level systems, and there are currently hundreds of PC/104-Plus processor and I/O expansion modules.
However, as embedded applications get more complex, and a single board is expected to do more, high-performance capabilities are welcomed. Many large chassis-based applications are being reduced to a small stack of PC/104-sized boards, creating huge savings for OEMs throughout a product’s life. Performance in this range is especially helpful for video-oriented applications such as frame grabbing, multi-channel video recording, image analysis, and real-time image enhancement, in addition to packet processing applications such as real time monitoring, filtering, or encrypting 1 GB or 10 GB Ethernet streams. Designers of higher-performance systems running applications such as these need to look to the newest PC/104 formats: PCIe/104 Types 1 and 2.
The formats support PCI Express (PCIe) busses running up the stack. This results in high-speed transfers between the processor board and the stack’s expansion boards. The transfer rate of the original PCI interface peaks at about 133 MBps. In contrast, a PCIe x1 (“by one”) link transfers data at about 250 MBps. For higher bandwidth applications, a PCIe x4 link provides a 1 GBps transfer rate. Both of these actually perform better than the numbers indicate, since each PCIe link is not a shared resource on a bus but an independent connection. Many PCIe links can be transferring data simultaneously, which multiplies the effective bandwidth.
Choosing the right Type
Both Type 1 and Type 2 boards stack up using the same high-speed “3-bank” connector. The majority of the signals on the 156-pin interface connector are identical between the types. By design, most PCIe/104 Type 1 and Type 2 boards can be mixed in the same stack.
For very high bandwidth applications the PCIe/104 Type 1 interface includes a single x16 link that supports data transfer at 4 GBps – about 30 times faster than a PCI bus. A Type 1 processor board combined with a Type 1 graphics processing board can move a lot of data.
Type 2 boards can accommodate quite a bit of bandwidth with their four x1 and two x4 PCIe links, but they also route additional I/O signals up through the stack. The I/O signals, such as USB 3.0 and SATA, allow plug-in (stack-up) add- on boards to operate without any additional cabling to get to these signals. For example, a PCIe/104 Type 2 SSD can be simply added to the stack without cabling to route SATA signals to it.
Designers should use a Type 1 processor board if they need a fast x16 link for lots of data; or use a Type 2 processor if their system does not need a x16 link, and can benefit from more available I/O signals in the stack (Table 1).
Not just for PC/104
The Consortium also supports the PCIe/104 interface on larger EPIC and EBX (Figure 1) format boards that can accommodate high-performance processors. Marrying these larger SBCs with PCIe/104 expansion can be used to create some very-high-performance systems that require no chassis or backplane.
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