PICMG and PISA Buses

The PICMG and PISA standards are alternative approaches to supporting both ISA and PCI boards in a single computer. Each of these dual-bus architectures provide you with the flexibility to use both PCI expansion cards and legacy ISA cards.

PICMG stands for the PCI Industrial Computer Manufacturers Group. This group released the "PCI-ISA Passive Backplane Standard" (commonly called the "PICMG" bus) in the mid-1990s. PICMG systems take a railroad-car approach: ISA and PCI slots are placed end-to-end, and the single-board computer (SBC, or CPU card) incorporates edge connectors for both slots. Your CPU interfaces with the ISA bus via the ISA connector and the PCI bus from the PCI connector. The combined length of the PCI and ISA slots means that PICMG SBCs must use a full-length expansion slot.

PICMG 1.3 adds support for high-speed PCI Express (PCIe) slots in place of older ISA slots. Industrial PC systems with passive backplanes can support the latest high-speed graphics and data acquisition cards alongside standard PCI cards. Generally full-length PICMG 1.0 cards can safely pull up to 227W from the backplane. New PICMG 1.3 cards can draw up to 500W without damage, which not only makes them more robust, but also provides support for modern dual-processor systems and high-speed CPUs. The PICMG 1.3 bus carries all the signals for up to 16 traditional PCI cards and five or more PCIe cards -- far beyond what motherboard systems allow. You can design powerful systems with up to 19 expansion-card slots.

PISA ("PCI and ISA bus") grew out of the EISA standard. PISA takes an upstairs-downstairs approach. Pins on the PISA-bus connector are longer than either PCI or ISA pins, The upper pins carry ISA signals, while the lower pins carry PCI signals. This allows for SBCs that are roughly half the length of PICMG SBCs. In fact, our shorter MicroBox™ PC systems all use either ISA or PISA SBCs to achieve their shorter lengths. Unused PISA and PICMG slots may be used as ISA expansion slots because both connectors have all of the ISA pins in the standard positions for exactly that reason.

PISA-bus backplanes and SBCs can potentially provide you with a highly compact PC. PICMG backplanes and SBCs require more length, but there are a greater variety of them to choose from.