Technical Library

Windows 3.1/3.11 Dial-Up Networking with ISDN

I/O Card - 16550 UART Chip

16550 UARTs: . . . UART is short for "Universal Asyncronous Receiver - Transmitter". All serial devices, such as serial modems, use a UART interface chip to communicate with your PC. External modems connect to your PC using a serial cable hooked to one of your PC's UART-based serial ports, while internal modems have a UART-based serial port on-board.

It is important that you know a little about UARTs in order to avoid problems in setting up your system to efficiently use your modem. Basically, UARTs convert parallel data from your computer into a serial data stream, and vice-versa. Most serial ports that come with PC's (especially non-Pentium systems) have a 8250 or a 16450 UART. While these chips are capable of passing data at rates much higher than is necessary to keep a 28.8 modem busy, they have only a single character buffer and lack onboard flow-control. The problem is that on many systems Windows may not be able to fetch characters from the UART fast enough to prevent the next incoming character from overwriting the single-character buffer, resulting in a "comm overrun" or "CRC" error.

Run MSD.EXE, the Microsoft diagnostic program which comes with current releases of both DOS and Windows (use the one with the newest file date) , to see if you have a 16550 UART. EXIT Windows before running, as executing MSD in a DOS window can give bogus results. If MSD shows you don't have a 16550 variety of UART, you should upgrade your serial port hardware. You can buy a new serial card which contains a 16550 UART for about $30 plus or minus. Look for UARTs labeled 16550A, 16550AF, or 16550AFN; some of the early chips with just "16550" had problems.

Windows 3.xx - Limited COMM.DRV Driver

Download & Install CyberCom...This is for Windows 3.xx CyberCom is a direct replacement for the standard Windows Communications Driver (COMM.DRV).

• Transfer at up to 115,200 KB with a 16550 serial port chip
• Great for V.FAST and Voice modems that require 57,600 Kb
• More reliable Transfer with less overhead on your system - Better background operation.
• Fewer (if any) over/under runs.

To obtain the CyberCom Driver, do a search using your favorite search engine.

Installation Instructions

• Copy CYBERCOM.DRV into your Windows\System directory.
• Edit the Windows\SYSTEM.INI file and change comm.drv=comm.drv to comm.drv=cybercom.drv.
• If you previously have not taken advantage of the 16550 installed in your computer, then ensure that the Windows\SYSTEM.INI file has the following information: [386Enh] COMnFIFO=1 COMnBUFFER=1024 COMBOOSTTIME=10 where n is the number of the COM port.
• Start or Restart windows.