Solution:

PCL-746+ rev. A1 supports standard COM1-COM4 when set to standard mode. With this mode, COM1 and COM3 share IRQ4, COM2 and COM4 share IRQ 3. PCL-746+ rev. B1 doesn support IRQ sharing in standard mode and user cannot set the same IRQ for 2 ports, therefore this version cannot set to standard COM1-COM4 like rev. A1. If user needs to work with this mode, PCL-746+ rev. B1 still can be used with small modification. 1. Remove the original 0 ohm resistors at D7, D8, D9, D10 then use four 1N4148 SMT diodes to replace them. 2. Add 1K ohm SMT resistors to R8 and R9. After this modification, user can set COM1/COM3 to the same IRQ number, and COM2/COM4 to the same IRQ number