PCIe/104 Type 2 at first glance

PCIe/104 is implemented by a pair of high-speed surface mount connectors. This paradigm is a major change from the through-board stacking connectors used by PC/104 and PCI-104, and it opens some exciting new possibilities. There is no requirement for the connector on the top of the host to be electrically connected to the connector on the bottom of the host. That means there can be a PCIe/104 bus going up and a completely separate PCIe/104 bus going down. That is a total of eight PCI Express x1 links, two PCIe x16 links, and four USB, doubling the number of peripheral boards. The best part is that because the specification requires all peripheral boards to be universal, all existing peripheral boards will work either above or below the CPU. All of this is already possible without a single change to any specification.

Introducing PCIe/104 Type 2

On the existing PCIe/104 bus, now referred to as Type 1, connector Bank 1 contains four PCI Express x1 links, 2 USB 2.0 ports, SMB, and some control signals. Bank 2 and Bank 3 contain only the PCI Express x16 link. In contrast, Type 2 maintains Bank 1 completely and maintains the same power, ground, and control signals on Banks 2 and 3 to ensure there will be no conflict with any existing PCI Express x1 or USB peripheral card.

Changing the PCI Express x16 link only affects PCI Express x16 peripheral cards and gives quite a few pins for Type 2. Bank 2 has PCI Express transmit signals, and Bank 3 has PCI Express receive signals. This convention, as well as all power and ground pins, was kept to ensure that there are no destructive consequences of improper stacking.

Type 2 adds two SATA ports to facilitate hard drives in the PC/104 stack. USB 3.0 caters to the future and is prominent in most companies’ road maps. Lane shifting on the SATA and USB 3.0 once again means universal peripheral boards. SATA and USB 3.0 are high-speed signals that require a connector with good signal integrity such as that offered by the PCIe/104 connector. LPC adds support for legacy devices, and many applications need a battery to back up the real-time clock. Two PCIe/104-compatible PCIe x4 links round out the connector. Table 1 shows the features and complementary nature of Type 1 and Type 2.

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Table 1: A comparison of PCIe/104 Type 1 and 2

PCIe/104 Type 2 advantages

All peripheral boards using a PCI Express x1 link, PCI Express x4 link, or a USB 2.0 link on a PCIe/104 Type 1 bus will work on a Type 2 bus without changes to the existing peripherals. This allows peripheral module manufacturers to create one board that works on either bus.

Since Type 1 and Type 2 can be connected together mechanically, it is very important to note that neither board will be damaged if a card is inadvertently plugged into the wrong configuration. The bus is designed with the same power/ground connections and similar transmit/receive connections and will hold the host processor in Reset mode if incorrectly connected. Users can be confident that the boards from all manufacturers will play nicely together.

Host processors can have both connector pinout configurations on one board for additional functionality. For example, Type 1 on bottom and Type 2 on top will support a stack configuration from bottom up comprising power supply, PCI Express x16 video card, CPU, and SATA hard drive. To create compact, expandable SBC designs where PCI Express x16 is not required but the additional functionality of SATA, USB 3.0, and LPC is mandatory, a host can use Type 2 on top and bottom. This might be a useful configuration for embedded processors like Intel’s Atom. Just like Type 1, Type 2 provides mechanical backward compatibility and bridging to PCI-104, PC/104-Plus, and PC/104 modules.

PC/104 Consortium 916-270-2016 info@pc104.org www.pc104.org