Solution:

PCL-746+ in enhenced mode shares the same IRQ for all 4 ports. The communication drivers of Windows 3.1 and Windows 95 supports IRQ sharing, but Windows NT 3.51 doesn't. So PCL-746+ can only use 1 port under Windows NT. We have developed a new Windows NT 3.51 communication driver which supports IRQ sharing. With the new driver, PCL-746+ can use all 4 ports under Windows NT 3.51. Please contact us for this driver if you need it.