New connector specification saves space, adds flexibility to stack PC/104 cards

February 19, 2015 // By Graham Prophet
The PC/104 Consortium has announced the latest revision of the PCI/104-Express & PCIe/104 Specification, Revision 3.0, that provides an additional option called "OneBank".

The PCIe/104 OneBank uses a smaller, lower-cost bus connector which is compatible to the full size PCIe/104 connector currently in use. It allows designers to stack boards using a complementary format that frees up PCB real estate for additional components as well as potential cost savings.

The history of the stackable PC/104 architecture is tightly coupled to the concept of building on the foundation of the PC market as defined by the major processor and peripheral chip manufacturers. This began with the ISA bus (PC/104), then the ISA and PCI buses together (PC/104-Plus), then the PCI bus only (PCI/104), and then the PCI and PCI Express buses together (PCI/104-Express). For additional room on a module, the PCIe/104 format removes the PCI bus. This roadmap allows maximum use of the resources and economies of scale of the PC community in both hardware and software development. It also allows maximum backward compatibility with minimal stacking overhead, if any, and allows future bus advancements as technologies develop and mature.

The origin of the OneBank option to the stackable PCIe/104 bus comes from the desire to minimise the resources needed for a given application. The standard PCIe/104 bus allows for maximum bus feature flexibility. It consists of a three bank connector with 152 pins and is available as either a Type 1 or Type 2. Both Type 1 and Type 2 have a common feature set and pin assignments which include: Four x1 PCI Express Links, Two USB 2.0, ATX power and control signals: +5V Standby, Power supply on, Power Good, Power: +3.3V, +5V, +12V, SMBus. Type 1 has the common feature set plus one x16 PCI Express Link that can be configured as two x8 Links or two x4 PCI Express Links dependent on the host. Type 2 has the common feature set plus two x4 PCI Express Links, two USB 3.0, two SATA, LPC Bus, and RTC Battery. With speed capability up to PCIe Gen 3, developers are given plenty of bandwidth for the future.

The OneBank connector concept consists of removing two of the three "banks" of the standard PCIe/104 connector, resulting in a 52-pin connector as opposed to the full-size 156-pin connector. The OneBank connector is positioned so that it will plug into the bank 1 of the standard PCIe/104 connector. The signals of the OneBank include the same four x1 PCI Express Links, Two USB 2.0, ATX power and control signals: +5V Standby, Power supply on, Power Good, Power: +3.3V, +5V (reduced current) as found on the first bank of the standard PCIe/104 bus making them plug-in compatible. This preserves the stackability and compatibility of PCI/104-Express and PCIe/104 modules along with the new OneBank modules. By removing two of the banks, 0.513 in² of PCB real estate on each side is freed up. And with speed scales up to PCIe Gen 3 on the PCIe/104 bus, developers are given plenty of bandwidth for the future even with just four x1 PCIe links.