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.
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.
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