Expanding Portable Systems

Founded in 1990, the Personal Computer Memory Card International Association (PCMCIA), of which Quatech is a member, developed a set of standards by which additional memory could be added to portable systems. It soon became apparent that this same interface could be used to add I/O devices and hard disk drives as well, thereby dramatically increasing functionality of laptop computers. Today, just about any device available for desktop computers using an ISA or PCI bus is also available with a "PC-Card" interface for use with laptop computers, and in some cases, handheld machines.

Physical Characteristics

The PCMCIA specification 2.0 release in 1991 added protocols for I/O devices and hard disks. The 2.1 release in 1993 refined these specifications, and is the standard around which PCMCIA cards are built today.

PCMCIA cards are credit card size adapters which fit into PCMCIA slots found in most handheld and laptop computers. In order to fit into these small size drives, PCMCIA cards must meet very strict physical requirements as shown in the figure below. There are three types of PCMCIA cards, Type I generally used for memory cards such as FLASH and STATIC RAM; Type II used for I/O peripherals such as serial adapters, parallel adapters, and fax-modems (this is the type of card Quatech manufactures); and Type III which are used for rotating media such as hard disks. The only difference in the physical specification for these cards is thickness.

Card & Socket Services

Functionally, a PCMCIA card can perform any memory or I/O operation so long as it adheres to the PCMCIA interface structure. As shown in the figure below, PCMCIA is a tiered system which uses a set of device independent drivers to integrate any type of PCMCIA card into the host system. Socket Services, the lowest tier in the architecture, provides a universal software interface for the PCMCIA sockets themselves. Socket Services manages all the sockets installed in a system so that resources can be properly allocated. It is also the means by which individual cards access registers on the host system. Socket Services can be added to a computer as a device driver, or it can be built into PC BIOS.

Directly above Socket Services in the hierarchy sits Card Services. Card Services is an application programming interface (API) which permits multiple software programs to work with multiple PCMCIA cards. For instance, Card Services will allow both Internet applications and fax applications to use an installed PCMCIA card modem. Like Socket Services, Card Services can be implemented as a device driver. It can also be built into a computer's operating system, as it is in Windows 95/98/NT/2000/XP and OS/2.

16-Bit PCMCIA

PCMCIA specification 2.1 provides for a 16-bit bus interface, has a maximum clock speed of 10MHz and is capable of speeds to 20Mbps. The 2.1 spec. does not provide for bus mastering, DMA, or multiple interrupts, (however, Quatech's interrupt sharing software drivers allow sharing the one interrupt among multiple I/O devices). While PCMCIA provides only a minimal performance improvement over ISA, and does not come close in speed to PCI, it does provide for considerably more flexibility than either of the others.

The two most important features of PCMCIA are its Plug and Play and Hot Swapping capabilities. As with PCI, PCMCIA cards are truly Plug and Play--you simply insert them, and instructions coded into chips on the card provide the information a host needs to configure the cards and appropriately allocate resources. Not only are there no jumpers or switches to set, users never even see the inside of a PCMCIA card. It is simply inserted into the drive, and the system does the rest. (An open PCMCIA card is pictured below, to show what you've been missing.)

Quatech's QSP-100 PCMCIA card Uncovered

This configuration procedure, along with the fact that PCMCIA cards are not connected directly to the motherboard, but are easily inserted into and ejected from a PCMCIA drive, allows the cards to be hot swappable. This means that the system need not be shut down then rebooted to add, remove, or exchange cards. Thus, you could insert a PCMCIA scanner, scan a drawing of your newest board layout, then remove the scanner and insert a modem and e-mail the scan to a manufacturer for mass production. While this might not be very important for desktop PCs with large numbers of expansion slots, it is vitally important for laptops with limited resources and usually only two PCMCIA slots. It becomes even more important for handheld computers which often have only one PCMCIA slot and one serial port.

32-Bit CardBus

In 1995 the PCMCIA 2.1 specification was enhanced to provide for 32-bit operation. The new architecture, called CardBus, was closely based on the PCI bus, and strove to provide the same improvements over the 16-bit PCMCIA card as PCI did over ISA. As such, CardBus provides for 33MHz operation and correspondingly increased data transfer. It also introduces DMA and bus mastering to PCMCIA based systems, which can markedly increase performance. Realizing that there are still many 16-bit PCMCIA card peripherals in the marketplace CardBus is fully backward compatible with the older card design.

Because of this backward compatibility, Quatech has decided not to redesign our serial data communication PCMCIA cards for CardBus or any of the other newer PCMCIA-based busses such as CardBay, as doing so would limit the number of systems that could use our cards. As discussed with PCI, because of the limitations imposed by serial and parallel transfers, there would be no noticeable performance gains for Quatech serial cards under CardBus.

PCMCIA for Data Communication

Though PCMCIA card use is not limited to portable computers, (see PCMCIA drives for desktop PCs) there are few instances where it is the best choice for data communication in desktop computers, however it can be a very useful choice for sharing peripherals that were purchased for primary use in a laptop such as a wireless modem or a scanner. In desktops, PCMCIA is better suited for adding extra storage space via hard-disk cards, or transferring large files from portable systems. However, for laptop and handheld computers, PCMCIA provides a way to connect a varied array of peripherals to the system, and to share those devices with a desktop computer.

Clearly there is a size advantage to PCMCIA for portable applications. The cards are small, light, and have low power requirements. They are an ideal interface choice for peripherals that have been scaled down for portable use. Further, the ability to Hot Swap PCMCIA cards provides for the flexibility needed to use multiple peripherals with only one or two slots. USB which also provides Hot Swapping is another alternative for portable applications. However, to use USB the peripheral devices in your system must be replaced with bus specific devices--an expensive prospect. Further, many USB products must be powered by the computer itself, thereby reducing the time a laptop can function on battery alone. Or, if too much power is required, they must be plugged-in, making them less attractive portable solutions. So if cost and power conservation are your primary concerns, PCMCIA is still the best, most flexible choice for portable applications.

There are USB adapters available, (such as Quatech's USB to Serial adapters) that perform the same function as serial PCMCIA cards--essentially permitting standard RS-232 or RS-422/485 peripherals to be connected to a PC that lacks native serial ports. Such products exist for parallel ports as well. With parallel ports in particular, (but in some cases with serial ports ), PCMCIA is a much more reliable solution. Many software applications designed to work with a computer's native ports have much more success using a PCMCIA based parallel port than a USB-based parallel port. This may be because PCMCIA was based on a traditional plug-in board bus, and thus the add-in ports implemented via PCMCIA are more similar to a native port than those implemented via USB which has a completely different bus architecture.

PCMCIA Specs
Bus Clock Signal		10 MHz
Bus Width		16-bit
Theoretical Max. Transfer Rate		20 Mbytes/sec (160 Mbits/sec)
Advantages		Ideal for portable systems, hot swappable, Plug & Play
Disadvantages		lower speed , needs special drive for use in desktop PCs

Click here to see how PCMCIA compares with other busses.