Solution:

The below table shows the IRQs default setting on PPC-100T COM port and LPT1. The problem of failure on LPT1 set to SPP is because the SPP is using interrupt channel 5, but it conflicts with setting IRQ 5 on COM3 and COM4. However, when LPT1 set to EPP, (using DMA channel) it can work properly. The solution is set the COM3 and COM4 to IRQ 10 to release IRQ 5 for LPT1 port. IRQ I/O Address COM1 4 3F8 COM2 3 2F8 COM3 5(default) or 10 3E8 COM4 5(default) or 10 2E8 LPT1 5 278