In article <[log in to unmask]>, [log in to unmask]
says...
>
><<I believe the problem is an overlap in IRQ addresses.  Doesn't COM3 use
>the same address as COM1and Com4 as Com2 (normally)?>>
>
>Yes; the "standard" settings are
> COM1: 0x3F8, IRQ4
> COM2: 0x2F8, IRQ3
> COM3: 0x3E8, IRQ4
> COM4: 0x2E8, IRQ3
>
>Since the PC/AT architecture uses edge-triggered interrupt lines, it is
>not possible to share an IRQ line between two devices.
>
>However, it is at least as likely that he is having a problem with his
>I/O address; for unknown reasons, IBM used an I/O address range that
>includes 0x2E8 when they designed the 8514/A video adapter. Since there
>are a lot of video adapters that support this interface, including such
>popular items as almost all ATI adapters and all current S3 chips, there
>are a ton of video adapters out there that won't work with a COM4 device,
>even if the user selects an unused IRQ line. Yuk.
>
>Steve Dirickson         WestWin Consulting
>(360) 598-6111  [log in to unmask]
I had the same problem on my PC when I configured my modem on COM4. I called
Microsoft Tech support and diagnosed that I had a case of "sliding IRQ"!  What
the  net of all this is:  You cannot have your modem configured on COM4 if you
do not have a physical COM3 device configured - do you have a COM3 device?  If
not try setting your modem to COM3 and see if that works.
 
Dennis