Potential Problems Using the ASI PC Single-Switch Interface Cable with WinSCAN or SS-ACCESS


WinSCAN or SS-ACCESS! reports that it can not locate the switch interface cable.

The most likely reason is that the SS-ACCESS! single-switch PC serial interface cable is not plugged into one of the computer's serial ports - i.e., COM 1, 2, 3, or 4. Check this first. A second reason may be that the SS-ACCESS! cable is plugged into a serial port that has been disabled (usually by the BIOS setup procedure accessed during the power-on startup sequence). A third reason may be that the cable is plugged into a port already in use by another serial device, such as the mouse. For solutions to these situations, continue reading below.

WinSCAN or SS-ACCESS! does not warn that the switch interface cable is missing, but pressing the external switch when the scanning display window is open and running (or when the Test Switch... dialog box is open) does not appear to do anything.

WinSCAN appears to conflict with the mouse or with a modem (or both). One or more of the three devices won't work together.

Hardware conflicts are probably the most common problems encountered when new devices are added to a PC computer. Moreover, in the field of assistive technology and control access, often the computer and/or peripheral equipment in question are not the very latest models and/or have already been adapted in a variety of ways (e.g., other hardware add-on devices, operating system software modifications, terminate and stay resident (TSR) programs configured to load and run automatically, etc. This makes the job of installing new equipment and software more interesting, to say the least.

Each peripheral device connected to your computer should be attached to a specific I/O port. All PCs are equipped with a variety of ports for attaching hardware. Examples include serial ports for mice, switches, and modems; parallel ports for printers; game ports for joy sticks; keyboard buses for keyboards; IDT or SCSI ports for hard disks and CD-ROM drives; etc. Typically, there are two serial ports (COM1 and COM2) and one parallel port (LPT1) built into the computer on its motherboard. There also should be provision for adding two more serial ports (COM3 and COM4) and possibly another parallel port (LPT2) by plugging add-on devices into internal card slots. Physically connecting two devices to the same port normally is a sure recipe for failure.

The microprocessor in your computer works on only one task at a time. When a peripheral device such as a modem needs the processor to do some work, it sends an interrupt request (IRQ) to get the processor's attention and an instruction telling the processor what to do. When the processor receives an interrupt request, it stops the task it was performing, executes the new instruction, then goes back to the original task. Most peripheral devices do not like to share anything. If two devices are set up to use the same IRQ, one or the other (or both) may not operate reliably. The only way to insure reliable operation is to give each device its own IRQ number.

In addition to the IRQ, each I/O port needs to be assigned a unique I/O port address. This address is a location in computer memory used to exchange data between the computer and the device. If two devices are set up to use the same I/O address, the result is similar to the IRQ conflict; neither device will work reliably.

It is relatively easy to understand and insure that each device is plugged into its own port. For instance, the SS-ACCESS! cable can plug into COM1 and your mouse can plug into COM2 (or vice versa). Normally, the computer's BIOS itself is set up properly to handle the I/O port addresses. For example, the standard addresses for COM1 and COM2 are 3F8 and 2F8 respectively. (See the chart below for all four standard settings.) The problem arises over the IRQ assignments, particularly the IRQ numbers assigned to the serial ports when more than two serial devices are installed.

Standard Serial Port Settings
Port
I/O
IRQ
COM1
3F8
4
COM2
2F8
3
COM3
3E8
4
COM4
2E8
3

Notice that the standard COM port configuration assigns COM1 and COM3 to IRQ4 and COM2 and COM4 to IRQ3. In general, two devices should not share the same IRQ simultaneously. For example, with the standard settings, you will have problems if you have a serial mouse on COM1 and a modem orSS-ACCESSon COM3.

Many of the devices you can install in your computer require an IRQ, but there are very few IRQ numbers to go around. You need to insure that every device added to your computer is configured to have a unique IRQ and a unique I/O address (and is plugged into a unique port). The more devices added, the trickier this becomes. The best way to avoid a conflict is to know the port, I/O address, and IRQ used by every device in your computer before adding a new device. Examine each card, read each manual, or call the dealer or manufacturer to determine the default card setup and how to change these defaults if necessary.

With WinSCAN or SS-ACCESS!, hardware conflicts arise most frequently when an internal modem, serial interface card, or other serial peripheral device is installed in the computer in addition to the mouse and the switch interface cable. In general, the solution is either to (1) remove the modem entirely (and enable the built-in COM1 or COM2 port if it was disabled to use the modem) or (2) reconfigure the modem to use another COM port and IRQ.

Microsoft, Inc., provides a good diagnostic program for the latter purpose called msd.exe. This program normally is located in the DOS directory on your hard drive. Run msd (from DOS, before starting Windows) and examine the current COM port and IRQ settings. Then refer to the modem documentation for the correct procedure to reconfigure its COM port and IRQ settings, usually by flipping DIP switches or repositioning small jumpers between pins on the modem card itself.

Modems use IRQs to signal the computer that data has been received or sent. Insure that no other device is using the same IRQ as the modem. If the SS-ACCESS! cable is in COM1 and the mouse is in COM2, then both of these ports, their I/O addresses, and IRQ4 and IRQ3 are in use. On many PCs, IRQ5 is assigned to LPT2, the second printer port. (Run msd to check this.) If a second printer is not in use, IRQ5 might be a good choice for the modem.

After reconfiguring the modem (e.g., for COM3 IRQ5), if you plan to use Windows communications software, you also must reconfigure the COM port in the Windows Control Panel. After opening the Control Panel, select the "Ports" icon. Next, select COM3. Then click the Advanced button. Change the "Interrupt request line" to the number "5" (assuming IRQ5 is not used by a network card, CD-ROM or other device).

Some PCs and serial port adapters support "IRQ-sharing", the ability to have multiple ports using the same IRQ. This is common on MicroChannel (MCA) bus architecture PCs, such as IBM PS/2s, and EISA bus PCs. Windows 3.1 supports IRQ-sharing serial adapters. To use these properly, be sure the IRQ values are set correctly using the Control Panel Ports/Advanced dialog box. Also, if you do not have an MCA or EISA PC, you must set COMIrqSharing=TRUE in the [386Enh] section of your system.ini file.

If you decide to use COM4 for your modem, be sure to use the Control Panel Ports/Advanced dialog box to make sure that Windows knows the correct port address value and IRQ settings for your COM4 port. The address for COM4 is not usually specified in the BIOS data area of most PCs because it is not a standard port. Many serial adapters use a COM4 default base address of 2E8. Unfortunately, this address value conflicts with some peripherals, including 8514/A video adapters and certain network adapters. To work around this problem, reconfigure one of the peripheral adapters to use a different address. Then use the Control Panel Ports/Advanced dialog to reset the address value for COM4. If you do not have a COM3 on your computer and have set the modem to COM4, the COM port may "slide over" to COM3. In this case, it may be necessary to add a line in your system.ini file as follows: COM3IRQ=-1

Although this discussion has been very technical, the steps to solve most conflict problems involving SS-ACCESS!, the mouse, and a modem or serial interface card are not difficult to perform.

  1. Run the msd.exe program (from DOS before starting Windows). Check which COM ports are enabled and whether IRQ5 is free for use (e.g., currently assigned to a non-existent LPT2 port).
  2. Turn the PC off. Refer to the documentation for the internal modem or serial interface card and reconfigure that card to use COM3 or COM4 and IRQ5 (instead of IRQ4 or IRQ3).
  3. Plug the card back into the computer. Attach SS-ACCESS! to COM1 and the mouse to COM2 (or vice versa).
  4. Restart the PC (using the BIOS setup program to re-enable built-in COM1 or COM2 if the port had been disabled previously to use the modem card).
  5. Run Windows. Use the Control Panel Ports/Advanced dialog box to check the port address value and reset the IRQ setting for COM3 or COM4 to match the modem configuration.
  6. Restart Windows. Check that the mouse works. Run a communications program to check that the modem works. Run WinSCAN to check that SS-ACCESS! works as well.

* A potential problem may result when a "mouse click" function is placed in the main scanning display window all by itself. Unlike other scan keys, these functions do not repeat automatically when the external switch is held closed. They act once and then the selector resumes scanning again at the first scan key position in the display. However, if the Scan Rate happens to be set at .5 seconds and there is only one key in the display and the user holds down the external switch, the "mouse click" function can be repeated too fast. This can lock up Windows. A slower scan rate is the solution.

* If the mouse buttons are swapped (using the Windows Control Panel), WinSCAN's mouse click functions will continue to behave normally. If a left click normally would be required for an action of some sort, the WinSCAN user should choose the Left Click scan key, even though the user would have to click the right button on the mouse to get the same effect.

* If there is no mouse plugged into the computer or for some reason Windows does not detect a mouse (e.g., the mouse.ini file in the Windows directory may point at the wrong serial port), WinSCAN will not permit the user to run a setup that includes mouse functions. It will run setups that include keyboard functions and the special Text functions. When Text Plus is selected, the Text Window will open and run but the scan will skip over the row or column that contains the mouse functions.


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