Solution:

Because OS/2 doesn't support IRQ sharing. you'd better set different IRQ for each port. There's no special COM port driver for OS/2, so you can use default device driver "COM.SYS" to add extra COM port by edit CONFIG.SYS like the following example: DEVICE=C:\OS2\COM.SYS (3,03E8,5,d,t) (4,02E8,10,d,t) This line will set up COM3 at IO 3E8 IRQ 5, and COM4 at IO 2E8 IRQ 10. Syntax: DEVICE=PATH\COM.SYS (COM, Address, IRQ, BAD_INT, FIFO) COM => comm port number ( 1 to 4 ) Address => base address of comm port IRQ => interrupt setting for comm port, make sure no conflict with other devices BAD_INT => what to do while bad interrupt i => ignore d => disable this comm port p => post to application FIFO => comm port supports FIFO or not t => true f => false