By the mid-nineties the PC industry had a fast, cheap, well-supported bus, and the
industrial world had mechanics that could survive a factory floor. CompactPCI, published in
1995, was the obvious combination: PCI signalling, Eurocard boards, pin-and-socket connectors
and a backplane built for hot swap.
What it borrowed and what it changed
Electrically a CompactPCI backplane is a PCI bus, which meant that silicon, drivers and
operating-system support arrived for free. Mechanically it is nothing like a desktop: 3U or 6U
Eurocards, a 2 millimetre hard-metric connector with 47 rows of pins, front-panel handles with
microswitches, and staged pin lengths so that ground mates first and the hot-swap controller
learns about a board before its signal pins touch.
Hot swap, properly specified
Hot swap was the feature that sold the format into telecommunications. The specification
defines three levels, from basic through full, and the useful one is full hot swap: the
handle switch tells the platform a board is about to leave, software quiesces the driver,
an LED confirms it is safe, and only then does the operator pull. Nothing about this is
implicit, which is why it works in a rack somebody else has to maintain at three in the
morning.
Where it sits today
CompactPCI was largely displaced in new telecom designs by AdvancedTCA and, later, by
plain servers. It survives in machine control, rail signalling and test equipment, and in the
CompactPCI Serial variant, which keeps the mechanics and replaces the parallel bus with
PCIe, USB and Ethernet links.