https://archive.org/details/manualzilla-id-5766186/page/1554/mode/1up?q=8514

8514/a and COM4 Conflicts
The 8514/a video adapter was designed to coexist with a VGA adapter. To
achieve this condition, the 8514/a uses a different range of addresses.
Some of these are 16-bit addresses that are located at 42E8h, 82E8h,
92E8h, A2E8h, and E2E8h. Unfortunately, many serial controllers decode
only the first 12-bits of the I/O port address and assume that calls to
x2E8h (like all of those listed above) are intended for the serial port
(that is, 2E8h) rather than the video card. Therefore, COM4 cannot be
used on most machines with an 8514/a compatible video card unless the
address of COM4can be changed on the serial card (usually via jumpers),
or the serial controller decodes all 16 bits of the I/O port addresses.
There is no other way to get COM4 and any 8514/a compatible display
adapter to coexist. Keep in mind that this is an issue with the serial
controller rather than the 8514/a video adapter.

ATI Mach, S3 Vision/Trio, and COM4 Conflicts
As you saw in the last section, 8514/a video adapters will often
conflict with COM4 because of poor I/O address decoding. ATI’s Mach
chipsets and S3’s chipsets are based on IBM’s 8514/a standard and
frequently suffer the same problems as the 8514/a video adapters.