The PMC and XMC architectures, explained

The mezzanine that adds functions to the host board: PMC combines PCI electricals with CMC mechanics; XMC adds the serial fabrics of VITA 42. Here you find formats, connectors and where the catalog starts.

What a mezzanine is: the board's raised floor

A PCI Mezzanine Card is a board built to IEEE P1386.1: PCI bus electricals on Common Mezzanine Card mechanics, mounted in parallel above the host board via connectors and standoffs.

Like a building's mezzanine floor, the PMC fits between two adjacent boards in the rack: a single card measures 74×149 mm and uses up to four 64-pin connectors — P1 and P2 for 32-bit PCI, P3 for 64-bit, P4 for user I/O — plus optional front-panel (bezel) connectors. The result: PCI compatibility in a smaller, more rugged package than desktop plug-in cards. Carriers come in Eurocard formats: VME, CompactPCI and, more recently, VPX — one PMC on 3U, up to two on 6U.

XMC is the VITA 42 evolution: it keeps PMC mechanics and adds the high-speed differential connectors P15 and P16 — 114 pins each, 20 differential pairs — for serial fabrics independent of PCI: PCI Express (42.3), Serial RapidIO (42.2) and other "dot" specs. PCI and fabrics can coexist on the same mezzanine, or either can be used alone.

Drawing comparing a single-width PMC module and an XMC module: same 74×149 mm, P11–P14 connectors versus P15/P16 — The same footprint under the same front panel — 74×149 mm for the single width — and the whole difference is in the connectors. PMC carries the four PCI rows: P11 and P13 for 32-bit, P12 for the 64-bit extension, P14 for user I/O (on the module the specification calls them P1–P4; P11–P14 is the carrier's first mezzanine site, the second is P21–P24). On XMC, P15 and P16 appear — the differential connectors of the serial fabrics — and the PCI rows stay dashed: they may or may not be there, because an XMC is not required to speak PCI.

The service: custom platforms and microracks How the engineering team works

What the specification says

The family, item by item

The PCI base, the serial evolution and the footprints.3 entries

PMC · IEEE 1386.1

PCI on CMC mechanics

It defines which pins carry which PCI signals and reserves P4's 64 pins for arbitrary I/O. The double format exists but is rare; the 1 mm connectors withstand shock and vibration better than a card edge.

XMC · VITA 42

Serial fabrics · dot specs

The 42.0 base defines mechanics and connectors; the "dots" map protocols: 42.1 Parallel RapidIO, 42.2 Serial RapidIO, 42.3 PCI Express, 42.4 HyperTransport, 42.5 Aurora, 42.10 general purpose I/O. The Samtec connectors are rated for 900 insertion cycles.

Footprint

Single · double · short

Single 74×149, double 149×149; short versions (139 mm) omit the PCI connectors. Power: up to ~100 W input, but air cooling in standard carriers handles ~15 W — beyond that, conduction takes over.

What a mezzanine carries

The catalog's real functional families: specialized I/O travels on mezzanines.1 tables

The catalog families

From the PMC and XMC modules category.

Family	What it brings to the host
Avionics buses	ARINC 429, MIL-STD-1553, 10 Mbit 1553
Analog and digital I/O	acquisition, counter/timers, motion control
FPGA	on-board processing and sensor processing
Communications	Ethernet and switches, serial, fieldbus
Graphics and storage	video compression, disks and memory