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How to Identify Ports on a Computer using DOS or Windows 3.x

Document relates to:   LapLink for DOS, LapLink for Win3.1, LapLink for Win95, LapLink Technical, LapLink Professional, LapLink 2000



My computer uses DOS or Windows 3.x. and I do not know which port to use for my LapLink serial or parallel cable. Can you help me?



LapLink supports the blue serial cable or the yellow parallel cable for connections in DOS or Windows 3.x. The computer port used by each of these types of cables is distinctive.

A serial port is male in gender, and has either 9 pins or 25 pins. The shape of the port is slightly D-shaped. A serial computer port may be used for a mouse, external modem, scanner, or other linking software such as for a personal data assistant (PDA).

On most computers, there are 2 ports and they can be marked as COM, SER, with a symbol representing 10101, or completely unmarked. It is also very common for computer manufacturerís to have installed 2 physical serial ports, but to leave one port disabled in the BIOS.

An internal modem or an Infrared (IR) port is named like other serial devices, although the ports are the phone jack or the red-glass-shielded light beam.

A parallel port is female in gender, and has 25 pins. The shape of the port is slightly D-shaped. This is the same port as where your printer attaches. On most computers, there is only 1 port and it can be marked as LPT or with a printer symbol.

Note: The LapLink parallel cable is proprietary, and must be used to connect over the LPT port.

For LapLink to connect, the following must be true:
  • the port is enabled in the computer's BIOS, and has a unique I/O address and IRQ setting
  • DOS and Windows correctly detects the port
  • the port is not used by another software program
There are two ways of identifying ports described here.
Using LapLink V for DOS
Using the DEBUG utility

Identifying ports using LapLink V for DOS
Note: These steps assume that LapLink V can use the mouse. If the mouse does not work, use the ALT key and the highlight letter. Press Enter to select a highlighted item.
  1. From LapLink V's Options menu, click Port/Modem Setup.
  2. Double-click COM1 to open the Current Settings for COM1 dialog.
  3. Click Reset. LapLink tests the port and finds if a mouse or a modem is attached. It also checks the serial port address and the assumed IRQ settings.
  4. If the Enable Port box is checked, the port is a serial port. If the Enable Modem box is also checked, then a modem was found.
  5. Click OK.
  6. Repeat for all other ports, including parallel ports. If Enable Port remains unchecked, the port is not physically present in the computer.
  7. When all ports have been reset, review the Port/Modem Setup dialog. Each port will read one of the following: Enabled, Mouse, Modem, or Unavailable.

Note: There should not be more than 1 serial/modem port and 1 parallel port enabled at a time. Remove the check mark from Enable Port to disable all unused ports.

If you are still unclear as to how your computer's ports are named, systematic trial and error is the best method to use. For example, two unlabeled ports are on the computer, and LapLink can enable COM2 and COM3.
  1. In LapLink, enable first COM2.
  2. With the cable attached to a port, see if LapLink connects.
  3. If so, then mark the port as COM2.
  4. If not, return to Port/Modem Setup and enable COM3 (disable COM2 first). See if LapLink connects.
  5. If LapLink fails to connect, move the cable to the other port and begin again.

If LapLink V is still unable to connect, continue with the remaining steps in this document.

Identifying serial ports using the DOS Debug command
This second method uses a DOS utility, preferably from the DOS command prompt with Windows shut down. We recommend that you use the table to track your progress.

Type of PortNumber Found on the Back of the ComputerAddresses Listed in DEBUG or BIOS
9/25-pin male serial ports
Internal Modem ports††††
Infrared (IR) ports
25-hole female parallel ports

Inventory the physical ports on the computer
  1. Count the number of serial devices in the computer. Include external serial ports, internal modems and IR ports.
  2. Enter this number in the respective box under the Number Found column.
  3. Locate all of the parallel ports.
  4. Enter this number in the 25-hole female box under the Number Found column.
To inventory the ports enabled in the computerís BIOS with the DOS DEBUG utility, follow these steps:
  1. Change the DOS directory on your computer.
  2. Type DEBUG and press Enter. The prompt will change to a hyphen (-).
  3. Type d40:00 and press Enter. You will see a display similar to this:
  4. Type Q and press Enter to exit the utility. The DOS prompt returns.
The important information is in the first line only. The first 8 characters are the line number. Next youíll see pairs of letters and numbers. They indicate each portís address. Each address consists of four numbers (as 2 pairs). For example, F8 03. The hyphen in the middle of this line separates the serial port addresses on the left from the parallel port address to the right.
  1. Count the number of serial addresses (to the left of the hyphen) and enter the addresses in the 9/25-pin male box under the Listed in Debug/BIOS column.
    Note: 00 00 is not an address. F803 is the address for COM1, and F802 is the address for COM2. The address for COM3 is E803 and E802 is the address for COM4.
  2. Review the addresses. If your computer has addresses for COM1, COM2 and COM4, you will need to follow the steps under Updating Windows Control Panel at the conclusion of this exercise.
  3. Count the number of parallel addresses (to the right of the hyphen) and enter the addresses in the 25-hold female box under the Listed in Debug/BIOS column.
Understanding what you've learned so far:
  • If the number of ports in both columns matches, the computer is correctly recognizing all of the ports.
  • If the number of ports does not match, the reason is one of these:
      -- a port may be disabled in the computer's BIOS.
      -- if you have added a serial/parallel add-on card to the computer, you may have 2 serial ports named the same. For example, 2 serial ports are named COM1. Debug shows only one address when this happens.
      -- a serial port and an internal modem are named the same.
      -- a port may have stopped working.
Matching serial addresses to serial ports, internal modems or IR ports
  • To determine if a port is an internal modem
    1. Exit Windows and type this line at the DOS prompt.
    2. Echo atdt11111>COM1
    3. Press Enter.
    4. If you hear a modem dial out, click, or the prompt immediately returns to C:\, then the port is a modem.
    5. If you have an error message display, then the port is a serial port.
    6. Repeat this line for each COM name recognized by the DEBUG command.
    7. If you find an internal modem, enter the address in the Internal Modem box under the Listed in Debug/BIOS column.
  • Using the computer's BIOS setup utility
    Note: Contact the computer's manufacturer for technical support with this step.
    1. See your user's guide to learn how to enter the computer's BIOS setup utility.
    2. Once in the utility, look for information about serial ports, infrared ports or internal modems. You may find this information in a 'peripherals' category.
      -- Any of these port types may have the address (F803) or the name (COM1) listed.
      -- You may also see Serial ports labeled as Serial A and Serial B.
    3. If you find the serial port address, circle it in the 9/25-pin male box under the Listed in Debug/BIOS column.
    4. If you find the internal modem address, enter it in the Internal Modem box under the Listed in Debug/BIOS column.
    5. If you find the IR port address, enter it in the Internal Modem box under the Listed in Debug/BIOS column.
  • For add-on serial and parallel cards
    Note: The computer's manufacturer, or other qualified hardware technician, may be need to assist you with this step.
    1. Locate the documentation for the serial/parallel port add-on board (if the board was not installed at the factory).
    2. Note: If no documentation exists, an alternative solution is to remove the card from the computer entirely.
    3. Turn the computer off and disconnect the power supply cable.
    4. Open the machine, and remove the card from the motherboard.
    5. Using the board's documentation, change the dip switches and/or jumpers so that the serial port is addressed with a unique address.
    6. Using the board's documentation, change the dip switches and/or jumpers so that the serial port is addressed with a unique IRQ setting.
    7. -- If the new address is COM2, use IRQ3.
      -- If the new address is COM3, try to use IRQ5 or 9. If not available, use IRQ4.
      -- If the new address is COM4, use IRQ5 or 9. If not available, use IRQ3.
      Note: Use Debug as a test to check the new settings. If your computer hangs after making a change an IRQ conflict is likely the cause.

Updating Windows Control Panel if your computer has COM1, COM2 and COM4
  1. Start Windows and open the Control Panel from the Main Group.
  2. Double-click the Ports icon.
  3. Click COM3 and click Settings, then Advanced.
  4. In Base I/O address, type the address that the DEBUG utility listed for the third serial port.
  5. In the Interrupt Request Line (IRQ) list, click 3 (the standard IRQ) unless you know that the port has been specially configured.


This Article can be found by searching for:

Keywords:   Cable: ParallelCable: SerialConnections: CableHow To: Windows

Platforms:   DOS, Win31


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Last updated: Friday, March 15, 2002

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