Introduction
Over the years, the ability to expand the capabilities of notebook computers has changed significantly. Originally expansion was handled through a card type called PCMCIA. It could handle a wide variety of peripheral capabilities including networking, I/O connections, storage and more. But as the speeds of the PCMCIA interface couldn't keep up with the computers, the PC Card or Cardbus standard was created that boosted performance. Once again notebooks have overcome the speeds of this interface and other ports such as USB and FireWire have reduced the usefulness of the current expansion card designs. This is where the new ExpressCard design comes in.
Interface Speeds
The primary purpose of the ExpressCard design is to increase the amount of bandwidth between the computer system and the expansion card. The original PCMCIA format was limited to the 16-bit ISA bus while the newer Cardbus used the 32-bit PCI bus. This gave the cards the typical bandwidth speeds of 10 MB/s and 33MB/s. While ExpressCard is much faster than this, the speed of the interface depends upon the implementation of the interface by the manufacturer.
ExpressCard interfaces can be based upon either the PCI Express or USB 2.0 interface standards. This gives the maximum throughput for the interface at around 300MB/s for PCI Express and 60MB/s for USB 2.0. The advantage of having ExpressCard being based around these two interfaces is for ease of integration into the latest computer systems. Of course, the style of interface will determine the overall possible throughput.
Two Sizes
Similar to how the there were multiple card types for PC Cards, Express card also comes in two sizes. Unlike the PC Cards that varied in height, the two ExpressCards sizes vary in width. Both cards are 75mm in length and 5mm in height The two widths are 54mm and 34mm. A notebook computer will refer to the type of slot it has by appending the size onto the end of the ExpressCard, such as ExpressCard/54 and ExpressCard/34.
While the slots have different widths, it should be noted that actual ExpressCard/34 modules can be used in an ExpressCard/54 slot. It is not possible though to use the ExpressCard/54 in an ExpressCard/34 slot. The reason for this is that both cards use the same interface that has 26 contacts. The 54mm card has a small 34mm section that sticks out of the back left side similar to the 34mm card.
For an image showing the size differences between a PC Card and the two ExpressCard formats, check out the PCMCIA Group graphics gallery.
So, why did the PCMCIA group design the card to have two sizes? The reason actually has to do with constraints of what the cards can be used for. While the ExpressCard/34 has the power and physical specs to handle pretty much any type of application, the card size does prevent it from being used for some applications. Two particular items are ExpressCard hard drives based around 1.8-inch drives and Compact Flash adapter cards. Both of these are too wide to fit into the 34mm width.
Sealed Box
One of the other design goals of the PCMCIA group for ExpressCard is to be used on something called sealed box computers. This would be any computer system that a user can expand without the need of opening up the computer system. Consumers already see this in notebook computers, but the group is focusing on this for desktop computer systems. Incorporating the ExpressCard slots into a computer system allows the PC to be expanded without opening the case. This can allow for much smaller computer systems to be built that require less technical expertise to expand.
While this is a very notable goal of the group, I think they may have overlooked a major issue with expansion. The ExpressCard slot is very small just like the PC Card formats in use today. The problem is that many cables that would need to interface with the ExpressCard are larger than the card. This would require that the ExpressCards either have dongles that attach to the card or have connectors that extend out of the card. A good example is Ethernet or video cables.
Conclusions
ExpressCard is a very welcome technology for portable computers thanks to its increased speed and reduced sizes. It is already finding its way onto some notebook computers from various manufacturers but the actual expansion cards are very limited right now. This will begin to change as more and more companies start integrating ExpressCard into their notebooks and phase out the older PC Card slots. Of course, the success of the USB and FireWire for peripherals may greatly limit the users need to upgrade the capabilities of their notebooks.
