The key difference is that XMC is an enhanced version of PMC incorporating high-speed serial fabric interconnects.
PMC (PCI Mezzanine Card) is a standard for mezzanine boards primarily using the parallel PCI bus for data transfer. XMC (Switched Mezzanine Card), defined by the VITA 42 standard, builds upon the PMC form factor but adds support for modern, high-speed serial fabrics.
Key Distinctions
Based on the provided reference, the fundamental difference lies in the type of interconnect technology used and the physical connectors:
- Interconnect Type: PMC relies on the parallel PCI bus. XMC utilizes high-speed serial fabrics.
- Connectors: XMC includes an additional connector (P5) that is specifically designed to carry these high-speed serial signals. PMC typically uses connectors J1/J2 and J3/J4 for PCI signals and I/O.
The Role of the P5 Connector
The defining feature of XMC, as highlighted in the reference, is the presence of the P5 connector. This connector enables support for various high-speed serial formats defined under the VITA 42 standard:
- PCI Express (VITA 42.3): Widely used for high-bandwidth peripheral connections.
- Serial RapidIO (VITA 42.2): A switched fabric often used in embedded systems and high-performance computing.
- Parallel RapidIO (VITA 42.1): Another RapidIO variant, though serial is more common in modern designs.
This means an XMC card can leverage the significantly higher data transfer rates offered by these serial technologies compared to the parallel PCI bus used by a standard PMC.
Comparing XMC and PMC
Here's a quick overview of the main differences:
| Feature | PMC (PCI Mezzanine Card) | XMC (Switched Mezzanine Card) |
|---|---|---|
| Interconnect | Parallel PCI bus | High-speed Serial Fabrics (PCIe, Serial/Parallel RapidIO, etc.) |
| Standard | IEEE 1386.1 (derived from PMC) | VITA 42 |
| Key Connectors | J1/J2 (PCI), J3/J4 (I/O) | J1/J2 (PCI/Legacy), J3/J4 (I/O), P5 (Serial Fabrics) |
| Performance | Lower bandwidth (parallel PCI) | Significantly higher bandwidth (serial fabrics) |
| Form Factor | Same physical size and mounting as XMC | Same physical size and mounting as PMC, but with P5 connector |
Practical Implications
Because XMC maintains the same form factor and often includes the legacy PMC connectors (J1/J2), some XMC cards can be backward compatible with PMC sites, potentially operating at the lower PCI speeds. However, to utilize the high-speed serial capabilities of an XMC card, it must be plugged into a carrier board or system slot that provides the VITA 42 P5 connector and supports the specific serial fabric (like PCIe) the XMC card uses.
This evolution allows system designers to integrate modules with much higher data throughput for applications like high-speed data acquisition, processing, or networking, while potentially maintaining compatibility with existing PMC infrastructure.
