Carrier cards and carrier boards have slots for other modules. Intelligent carrier devices have a local processor. Non-intelligent carrier devices, or slaves, connect the modules via a bus bridge to a larger host bus. Carrier cards and carrier boards are available in a variety of clock speeds and bus widths. The clock speed is the speed at which the microprocessor executes instructions. The bus width describes the number of bits that the bus can transmit at one time in parallel.
There are several types of carrier cards and carrier boards. PCI mezzanine cards (PMCs) use logical and electrical standards based on peripheral component interconnect (PCI) specifications. Some PMCs use the standard mezzanine card form factor of 3 x 5 inches, while others are smaller and denser. Several proprietary PMCs cards are currently available, and some are designed for use in rugged, industrial applications. Carrier cards and carrier boards also follow standards developed by the Personal Computer Memory Card International Association (PCMCIA) for small devices. These peripherals, or PC cards, are the size of credit cards and are used predominantly in laptop computers. PCMCIA adapters replace the COM port on a standard bus and translate PCMCIA signals into signals that the bus can use. Embedded technology eXtended (ETX) is a form factor for developing embedding systems that allows for easier designs by eliminating cables and connectors. The ETX-PC has a small footprint of 114 mm x 95 mm and a maximum thickness of 12 mm.
There are many form factors for carrier cards and carrier boards. 3U CompactPCI buses are 100 mm x 160 mm and have a bus width of 32 bits. 6U CompactPCI buses are 233 mm x 160 mm and have a bus width of 64 bits. PXI, an extension of PCI, is an open industry standard developed by National Instruments. 3U PXI cards are 100 mm x 160 mm and have a bus width of 32 bits. 6U PXI cards are 233 mm x 160 mm and have a bus width of 64 bits. VME Size-B cards also have a form factor of 233 mm x 160, while VME Size-C cards are larger at 233 mm x 340 mm. Another National Instruments standard, VMEbus eXtensions for Instrumentation (VXI), uses a form factor of 233 mm x 160 mm.
Carrier cards and carrier boards that follow Industry Standard Architecture (ISA) can handle 16-bit data transfers at a clock speed of 8 MHz. EISA, or Extended ISA, architecture is an improved ISA bus that runs at 8 MHz and is capable of 32-bit data transfers. Like ISA and EISA buses, PCI buses can be either full size or small form factor. Small form factor PCI buses enable PCI expansion in smaller mechanical packages. PC/104 cards are smaller than ISA bus cards and stack together to eliminate the need for a motherboard, back plane, and/or card cage. PC/104-Plus cards incorporate the PCI bus into the PC/104 form factor for faster data transfer.