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What Is PCMCIA; What Is CardBus?

Hardware: Multifunction DAQ (MIO)

Problem: I often read the terms "PCMCIA", "CardBus" and "PC Card". What do they mean and what are the differences between them?

What is the difference between "PC Card", "CardBus", and "PCMCIA"?

PCMCIA (Personal Computer Memory Card International Association) is a nonprofit trade association and standards body that promotes PC Card technology. In the past, cards were known as "PCMCIA Cards", but the industry now refers to products based on the technology as "PC Cards" and refers to the association as PCMCIA. "CardBus" and "PC Card" are two alternative types of PCMCIA interfaces on laptops, desktops, and PDAs. PC Card is a 16-bit interface, while CardBus is a 32-bit interface. The CardBus 32-bit interface supports higher data rates.

What are the capabilities of "PC Cards"?
PC Cards are the mobile equivalent of the 'old' 16-bit ISA cards, often referred to as "PC Card-16", "Legacy PC Card" or "R2 Card". They offer the same performance and resources as the ISA host bus, which they are connected to. This implies that these cards require a so-called Legacy-Interrupt (non-PCI interrupt). In general PC Cards use an I/O voltage of 5 V but the PCMCIA 2.x standard introduced a low-voltage version with 3.3 V. To prevent low-voltage cards from being inserted into a 5 V socket, low-voltage are keyed so they cannot be inserted into 5 V sockets. Most sockets can accept both 5 V and 3.3 V cards.

What is CardBus?
CardBus is the 32-bit version of PC Card technology. Enabled in the February 1995 release of the PC Card Standard, CardBus allows burst transfers up to 132 MB/s as well as busmastering. Other than PC Card-16, CardBus limits the voltage range to 3.3 volts (low-voltage). Although CardBus is not the same as PCI in all respects, the signaling protocols are identical. The pins of the PC Card interface are dynamically redefined once the nature of an inserted card has been determined (by querying the card immediately following its attachment). Most laptops today come with CardBus socket(s). CardBus sockets must be able to accept and operate both 32-bit CardBus and PC Card-16 cards. The adapter characteristics are set programmatically based on the property of the cards inserted into a CardBus socket. However, a low-voltage-only CardBus socket cannot power 5 V-only PC Cards.

CardBus cards are easy to identify as they have an extra gold colored metal strip on the end of the card. This is an extra shielding required because of the faster data transfer. Unlike PC Card slots, CardBus slots support Direct Memory Access (DMA) and Bus Mastering.

How does this affect National Instruments PC Cards?
All the existing PC Cards from National Instruments (as of September 2001) are PC Card-16 compatible, and should work in all notebooks with PC-Card or CardBus slots (as well as ISA-to-PC Card carriers).

There is one thing to take care of:
As the ISA bus is disappearing from the market, PCI-to-PC Card carriers for desktops are becoming more common. In such systems, there must be some means for routing a legacy interrupt from a PC Card-16 to the host. One common method for routing a legacy interrupt is via serialized IRQs. Some PCI carrier/motherboard combinations may not support this interrupt method, and thus the PC Card-16 won't work.

Related Links:
External Site: Microsoft developer information on CardBus
External Site: PCMCIA PC Card Primer


Report Date: 09/24/2001
Last Updated: 02/08/2010
Document ID: 2DN9MOHR

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