Quatech RS-422/485 asynchronous serial PCMCIA cards are an ideal solution for adding additional ports to full and half duplex multi-drop systems. Because they share electronic fundamentals with native bus architectures such as PCI and ISA, they function exactly like standard COM ports and can be addressed as such by most application software. In addition, their small size and the fact that the card itself can be left inside the computer at all times, make them practical choices for the most ported portable systems. Quatech RS-422/485 serial PCMCIA cards provide many unique features that make them particularly robust and easy to use. Each of the ports is configurable as RS-422 or RS-485, and on multiport cards any combination of RS-422 and RS-485 ports can be used. For high performance is heavy multi-tasking environments, the cards are implemented with 16750 UART's containing 64-byte FIFO's. This along with Quatech's special clock-multiplying feature enables data transfer rates as high as 921.6 kbps. (4-port cards still use 16550 UART's for standard 115.2 kbps speeds, but are due for an upgrade soon.) They also provide a unique Auto-Toggle mode by which the hardware itself can regulate data flow in Half-Duplex mode by only driving lines while actively transmitting--providing increased speed and convenience over the older software controlled method. In addition, the cards also conserve system resources with their low power requirements that maximize battery life. They also have selectable and shareable interrupts, further extending a portable or handheld system's limited resources. And their multi-platform operating support makes them the single practical choice for a diverse fleet of portable systems.