Several, popular mezzanine standards have emerged to support the configurability required by designers of COTS systems. Each mezzanine succeeded because it met specific targets in cost, size, and performance.

Industry Pack is a small, low-cost module widely used for various I/O functions on standard backplane boards of nearly every type, with four IP modules fitting on a standard 6U VME board. One of its two 50-pin connectors is used for module-specific I/O to the motherboard and then often out to front-panel connectors. The second connector provides a digital interface, supporting 16-bit transfers at rates of 4 MHz on a basic, single-width module.

Double-width modules are available to double the data bus to 32 bits and provide more space for circuitry. The digital interface also includes support for two DMA channels and two interrupts. Industry Packs are cost-effective for simple, low-bandwidth I/O functions and are available from a variety of vendors.

The MIX mezzanine bus was developed by Intel as a modular daughtercard standard for Multibus II systems. Pentek adopted this standard for VMEbus and offers baseboards and modules for both DSP and I/O interface functions. MIX module functions include a wide range of interfaces, including A/D and D/A converters, digital I/O functions, digital audio interfaces, SCSI controllers, digital receivers, PCM telecom transceivers, and prototyping modules. All of these functions can be combined for a nearly unlimited set of MIX subsystems handling application specific tasks.

The MIX bus includes a 32-bit data bus, 32-bit address bus, and a set of control lines, and is similar to a conventional microprocessor bus. The standard data transfer rate is 22 Mbps, but an enhanced version of the bus delivers over 40 Mbps.

PMC has gained the attention and support of just about every board manufacturer in the high-end system-bus community, first with VMEbus vendors and now increasingly with Compact PCI vendors.

The PMC specification is a combination of two standards: the CMC (common mezzanine card) format, which defines the physical aspects, and the PCI standard, which defines the electrical interface. The physical size of the PMC module is roughly 3 ę 5 inches. This size allows up to two modules to be attached to 6U VMEbus and Compact PCI boards. The module is attached to the carrier board using two, three, or four 64-pin compact connectors, depending on the application.

In addition to the four interboard connectors, the PMC specification allows for direct connection through the front panel of the VME board. A separate PMC front panel can protrude flush with the VME front panel through a cutout hole to accommodate any specialized I/O connectors required by the module. Most PMC modules utilize the 32-bit interface and are capable of moving data in block transfers at 132 Mbps.

Originally developed for Pentekęs VME quad ęC6x processor board, VIM provides a three-fold interface between the baseboard and the mezzanine card. The streaming 32-bit parallel interface utilizes a synchronous, bidirectional FIFO on the baseboard, capable of operating at up to 100 MHz. The synchronous serial interface supports two 100-Mbps full-duplex channels. The control/status interface provides microprocessor-like access for reading and writing to memory-mapped registers on the mezzanine for configuring and controlling module functions.

VIM uses a high-density pin and socket connector system, which delivers these three interfaces plus power using 160 contacts. Up to four VIM interfaces fit in a single 6U VME or Compact PCI board. VIM modules feature a front panel, which replaces a section of the front panel of the baseboard to accommodate application-specific connector types for a wide range of analog and digital interfaces.

In Table i, note that the major advantage with VIM is the high I/O bandwidth per board. PMC baseboards support a maximum of two PMC modules, usually with 32-bit interfaces and a single, shared PCI bus. VIM baseboards support four dedicated, independent VIM interfaces, each capable of up to 400 Mbps.