As I've noted before, the Hayes ESP board has a bug in its 16550 emulation
such that it will intr for every incoming character and NOT on the selected
FIFO level.  The fix for this (taken from the hayes ESP programmers guide)
is to enable the DMA bit.  (What this REALLY does, I have no clue as it 
doesn't do DMA.)

So for com.c users with a Hayes ESP board, add this line:
>#include <dev/isa/isavar.h>
>#include <dev/isa/comreg.h>
>#include <dev/ic/ns16550.h>

/* Enable DMA bit for Hayes ESP boards */
#define FIFO_ENABLE (0x01 | 0x08)

>#define COM_IBUFSIZE    (2 * 256)
>#define COM_IHIGHWATER  ((3 * COM_IBUFSIZE) / 4)

-j