Technical Note #1616

COM Port Troubleshooting using MSD

Application affected:	General	Date Created:	9/25/96
Version Number:	Windows 3.1	Last Modified:	9/25/96
File Date(s):	n/a	Created By:	Mike Rimov
Type of Note:	Technical Guide	Additional Info:

Keywords: Modem diagnostics troubleshooting windows 3.1 win com ports 16550 UART MSD

Summary:

This technical note provides a technician with an understanding of some of the inner workings of Communications Ports under Windows 3.1.

You will also learn some important details about Microsoft's Diagnostics (MSD) which is a tool that came shipped with every copy of Windows 3.1 (and some versions of MS DOS)

Details:

In order to get a picture of the COM ports found on a user's system, you need a point of reference. MSD is a tool that comes with every copy of Windows 3.1 or higher. From the DOS prompt, you can have the user run:

C:\> msd.exe

Have the user click on the "Mouse" label, to get information such as COM Port that the mouse is attached to. If the end user does not know which COM port their modem is attached to, then you will probably need to track down a working communications program that the end user has been running, and get the port/IRQ settings from that program.

If you select the COM Ports button in MSD, you will be given diagnostics information on the current ports installed on the end users system. On older systems you will probably be able to find the modem by looking that the row labelled UART chip used. High speed modems (14.4 to 33.6) use the 16550AF chip (or equivalent). Many built in serial ports in older machines use the 8250 chip. If you see only one 16550AF chip installed on the system, then this will most likely be the modem.

Microsoft Diagnostics Power-Tip: If the user has a more modern machine, and all of the serial ports are equipped with high speed UARTS (16550's), you can usually locate the modem among them by finding the port that has "Data Set Ready (DSR)" and "Clear To Send (CTS)" both set to "Yes".

NOTE: A word on reading the COM port settings within MSD. MSD does not accurately report the port number as far as communications programs are concerned. If the end user has COM1, COM2 and COM4 installed, MSD will list them as COM1, COM2, and COM3. It will skip the fact that the COM ports are not actually contiguously numbered. The only way to know which port MSD is actually displaying is to look at the Base address column and cross reference the information given with the port settings listed above. For example, if MSD listed COM1's base address as 3F8, COM2 as having a base address of 3E8, and COM3 as having a base address of 2E8, then what really exists in the system is COM1, COM3, and COM4.

Given this knowledge, you are now ready to track down a couple of possibilities. If the computer has two external serial ports and an Internal modem has been installed (thus bringing the COM port count officially to three), and only two COM ports are listed in MSD, then the chances are pretty good that the modem has been assigned the same port number as one of the existing Serial Ports in the system (thus, "Houston, we have a problem").

You should probably recommend at this point that the end user take his machine to a local computer repair center to finish tracking down the IRQ conflicts, since in 90% of the cases, this will require opening the system, and modifying jumper positions, etc.

©1996 Centerline Computers, All Rights Reserved. The information presented on this page, and all pages found on this web site, are for the use of Highway 101 Customers, Certified Highway Technicians, and Distributors. Any other commercial use is prohibited without prior written permission by Centerline Computers.