PCI Express has dominated PC and server markets, but its adoption in some embedded areas has been a bit slower. There are standards in place that complement the PCI standard boards that still make up the bulk of shipments in some of these spaces.
Backward compatibility and a plethora of legacy boards are the likely reasons for slower adoption in some areas like CompactPCI and stackable boards. Also, PCI often provides sufficient bandwidth for most existing applications. Still, PCI Express availability and features continue to make it a more desirable pathway.
CompactPCI Serial Emerges
PICMG’s CompactPCI now has PCI Express variants including CompactPCI Serial and CompactPCI PlusIO. CompactPCI Serial uses PCI Express to connect to peripheral boards. The standard specifies a star configuration with a system slot for the processor board at the center of the star. The system slot often is at end of the backplane.
Each peripheral board offers connections for a x1 PCI Express link, a SATA interface, an Ethernet port, and a USB port. A backplane will sometimes have two peripheral slots with x8 PCI Express links instead of a x1 link. Peripheral cards can have up to a x16 PCI Express interface. The system slot has a x16 PCI Express link and seven x1 PCI Express links plus eight SATA/SAS ports and eight Gbit Ethernet ports. USB 2.0 and 3.0 are both part of the interface as well.
Intel’s 2.1-GHz, quad-core, Core i7 powers MEN Micro’s F21P (Fig. 1) CompactPCI PlusIO single-board computer (SBC). Its PCI and PCI Express connections enable it to handle both types of peripheral boards. It also has four sets of PCI Express x1, USB, and SATA ports. A single-backplane Ethernet port is provided as well.
A typical backplane for a CompactPCI PlusIO system includes a set of CompactPCI peripheral board slots and another set of CompactPCI Serial peripheral board slots in addition to the CompactPCI PlusIO slot for the SBC. The backplane does not typically include a PCI Express switch.
Stackable PCI EXpress Alternatives
Stackable standards supporting PCI Express have been available from the PC/104 Consortium and the Small Form Factor Special Interest Group (SFF-SIG). They aren’t compatible, and they aim at different performance targets.
VersaLogic’s VL-EPMs-M1 multi-I/O expansion cards use the SFF-SIG’s SUMIT ISA standard (Fig. 2). The peripherals are accessed via PCI Express on the SUMIT connections along with other interfaces such as USB. The board exposes four USB ports, a pair of SATA interfaces, and a PCI Express Mini Card socket, which also supports mSATA flash drives.
SUMIT uses x1 and x4 PCI Express links. It can be paired with ISA and PCI connections, although ISA tends to be more common.
The PC/104 Consortium’s PCI/104-Express brings a x16 PCI Express link to the stack along with a PCI-104 connection. As with VersaLogic’s board, the non-PCI Express connection is usually a pass-through allowing older boards to be included in the stack. Likewise, the connectors with PCI Express support have connections for other interfaces like USB.
PCI/104-Express sports Type 1 and Type 2 interfaces with different characteristics. Type 1 has the x16 PCI Express interface and USB 2.0, while Type 2 has x1 and x4 PCI Express support plus USB 3.0 and SATA. SFF-SIG’s SUMIT has a smaller A and optional B connector. It also has PCI Express x4 and x1 links plus USB, I2C, SPI, and LPC connections.
Popular PCI Express Platforms
VITA’s VPX (VITA 46) and, to a lesser extent, VXS (VITA 41) have been very popular in areas other than PCs and servers. PCI Express is just one of the high-speed serial interfaces that these standards support. The XMC (VITA 42) mezzanine card standard supports PCI Express as well.
AdvancedTCA and MicroTCA are another combination where PCI Express has always dominated. Like VPX and VXS, PCI Express also has to contend with other serial interfaces like Serial RapidIO, Ethernet, and InfiniBand.
PCI and even ISA will continue to be supported, but the move to PCI Express is inevitable. Some tipping points are well past, while others are almost there.