The most common cause of comm lockups in Windows is a 16550-compatible chip
that isn't fully compatible. Typically this is seen on Pentium systems with
serial ports integrated on the motherboard, but it can also happen with
add-on cards, particularly "multi-I/O" cards based on popular SMC chips.
(See "PC Hangs While Running a Communications Application" at
* If you are running Windows for Workgroups 3.11, one fix is to download and
install the updated SERIAL.386 driver that implements a workaround for this
problem. But see warning below. (This fix wont work for standard Windows
3.1 or 3.11.)
* You should be able to completely solve the problem by turning off the UART
buffers. To do that, put the following in the [386Enh] section of your
where n is the number of your modem COM port (e.g., COM2FIFO=0). The
drawback is that you then won't get the benefit of the 16550 FIFO buffers.
(See "Do I need a 16550 UART? What is a UART?") For that you will need to
upgrade to Windows for Workgroups and apply the SERIAL.386 fix (see above),
or get a replacement UART.
WARNING: Although Microsoft specifically identifies the SMC 665 "multi-I/O"
chip as the problem, the author has personally verified that the problem
also exists in the SMC 666 "multi-I/O" chip (FDC37C666GT). Furthermore,
although the problem is reportedly fixed in current chip production, the
author has personally verified that the problem still exists in multiple
brands of I/O cards on the market as of August 1995. Worse, the author has
also personally verified that Microsofts updated SERIAL.386 is not fully
satisfactory: lockups are very rare but not entirely eliminated, and Windows
multitasking is adversely affected during certain communications operations.
(Negotiating a dialup PPP link with Trumpet Winsock 2.1F seems to trigger
the problem.) The only fully effective workaround is to disable the FIFO
buffers (as described above). FOR THIS REASON THE AUTHOR DOES NOT RECOMMEND
PRODUCTS THAT USE SMC "MULTI-I/O" CHIPS.
REQUEST: The author is looking for multi-I/O cards with real 16550 UARTs to
recommend as replacements for SMC-based multi-I/O cards. Any help would be
28800 Modem FAQ: http://web.aimnet.com/~jnavas/modem/faq.html