<<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]