PC/4e DigiBoard Intelligent Asynchronous Serial Communications Board

The DigiBoard MC/Xe / PC/Xe Intelligent Asynchronous Serial Communications Boards are designed for use with ISA and Micro Channel computers. The heart of the PC/Xe and MC/Xe boards is an 80186 microprocessor and 64K bytes of dual ported RAM, which relieves your computer of the burden of managing the serial ports. The computer can transfer large blocks of data directly to the memory on the board, then move on to other tasks while the board sends the data out the serial port one character at a time. Similarly, the board receives input data and stores it in buffers in its dual ported RAM, so the computer only needs to check periodically to see if data is available.

The dual ported RAM is memory which is accessible for read and write operations by both the board and the computer. To the computer, the dual ported RAM looks exactly like its own memory, and can be accessed by the same high speed memory referencing commands it uses for its internal memory. This means that a block of data that may take a number of seconds for the PClXe or MC/Xe board to receive or transmit to the outside world can be transferred between the board and the computer in mere microseconds.

The dual ported RAM is “mapped” into an unused area in the host computer’s memory address space (typically somewhere between 0C0000h and OEFFFFh-the area traditionally reserved for expansion board BIOS ROMs and dual ported memory). The PC/Xe or MC/Xe board can be set via software so that the entire 64K is mapped into the computer’s memory, or just an 8K “window”. Using the 8K window allows you to fit the board into a smaller space when it has to coexist with a number of other expansion boards, many of which also need memory address space in the 0C0000h-0EFFFFh area (the entire 64K is still used; it’s just divided into eight 8K segments which can be accessed by moving the window around).