What Is MCIO?

MCIO, short for Mini Cool Edge IO, is a small high-speed connector and cable system for routing PCIe lanes inside a computer. Instead of a card plugging straight into a slot, a cable runs from an MCIO connector on the motherboard to a drive backplane, a riser card or an accelerator elsewhere in the chassis.

Amphenol designed the connector, and SNIA’s SFF Technology Affiliate published it as the open standard SFF-TA-1016, so cables and boards from different makers work together. PCI-SIG has since used the same connector family for its CopprLink internal cable specification, which carries PCIe 5.0 and 6.0 signals over copper inside a system.

The name describes the design: a card-edge contact, as in a PCIe slot, shrunk to a fraction of the size, with the cable assembly built to keep signal loss low at 32 and 64 GT/s.

At PCIe 5.0 and above, signals lose strength quickly in ordinary circuit board material. Running lanes a long way across a motherboard to a slot or drive bay needs retimer chips or expensive low-loss board material. A shielded twinax cable loses far less over the same distance, so moving the long run into a cable is often the simpler design.

Cabling also frees the layout. The board maker does not have to fix every slot and drive bay in place, and one board can be built into several chassis with different storage and expansion. That is why MCIO connectors now sit close to the processor sockets on many current server and edge boards.

MCIO is one of several small connectors that carry PCIe over a cable. They look similar, but they are not interchangeable, and the cable must match both ends.

In practice, MCIO has taken over from SlimSAS on boards designed for PCIe 5.0, while SlimSAS remains common on Gen4 equipment. OCuLink is best known today as an external port for graphics docks on mini PCs and handhelds. Our guide to PCIe 5.0 explains why signal integrity tightens at each new generation, and the PCIe 6.0 explainer covers the PAM4 signaling that MCIO cables are rated for.

MCIO connectors are named by lane count. A 4i connector carries four PCIe lanes, 8i carries eight and the largest versions carry sixteen, with sideband signals for reset, clock and drive presence. The SFF-TA-1016 specification defines 38, 74, 124 and 148-position versions; without sidebands the same contacts can carry more lanes.

MCIO cables come straight or right-angled and in fixed lengths, and the impedance must match the generation; current PCIe cables are typically 85 ohm. A cable rated for PCIe 4.0 may train down or fail at 5.0, so match the cable to the board maker’s qualified list. Adapters from MCIO to PCIe slots, to U.2 drives and to SATA exist, but each must match the connector’s configured mode in the BIOS.

Most compact industrial PCs still use fixed slots and M.2 sockets, but cabled PCIe is spreading into rugged and edge designs where storage or cameras need to sit away from the board. Some of our industrial mini PCs bring PCIe Gen4 x8 out on a SlimSAS 8i connector for a PCIe camera, external NVMe or U.2 storage.

At the larger end, the Neousys RGS-9805GC rugged GPU server pairs a front-accessible U.2 tray with an internal SlimSAS port alongside its PCIe Gen5 slots. When a design needs MCIO, SlimSAS or OCuLink, the connector, cable and backplane have to be chosen as a set.

We specify the connectors, cables and backplanes as a matched set and check that every link trains at its rated speed as part of our standard build.