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COM4 Address Conflicts

As today's PC applications become both more graphic and communications intensive, users are finding that many systems will not work with COM4's address configured as 2E8h. Often the problem is blamed on the video card for using the COM4 address of 2E8h. Actually, video cards do not use 2E8h, but an 8514/A compatible video card does use addresses such as 42E8h, 4AE8h, 82E8h and others. These higher addresses require that more bits of the PC's address lines be decoded. Historically, in a PC/AT, only 10 address lines needed to be decoded, which would handle the address space up to 3FFh. Newer video cards require 16 address lines to be decoded, allowing additional address space to be used, up to FFFFh. The table below illustrates how two "video" addresses conflict with the COM4 address 2E8h.

COM4 Addresses

Note that only bits 0 - 9 are required to specify the 2E8h address. The "x" marking indicates that each of those bits is a "don't care" - meaning either a 1 or a 0 in those positions is acceptable. The problem is clear when the next two addresses (used by 8514/A compatible video cards) are examined. Each of these addresses duplicates the lower 10 bits of the 2E8h address, and only the higher bits are distinct. This means that anytime the PC attempts to communicate with address 82E8h, for example, a device residing at x2E8h will also respond. A very simple solution is not to use 2E8h for COM4. Most of B&B's serial cards allow any hex address up to 3FFh to be selected, allowing 2E8h to be avoided completely, even in systems with large numbers of serial ports. Note that the software being used with the port must be flexible as well. Fortunately, with the increasing popularity of more advanced operating systems, most communications software allows alternate addresses to be selected for COM4, avoiding the 2E8h conflict altogether.