<<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]