Solution:

The difference list about A1 and B1 version of PCL-746+ is shown as follows: 1. I/O address: A1 => one switch DIP for 4 ports standard mode => 3F8, 2F8, 3E8, 2E8 enhanced mode => base, base+08h, base+10h, base+18h B1 => four switch DIPs for 4 ports respectively, you can set I/O address for each port individually, no matter standard or enhanced mode. 2. IRQ: A1 => one IRQ jumper setting for 4 ports standard mode => 4, 3, 4, 3 enhanced mode => share IRQ B1 => four IRQ jumper settings for 4 ports standard mode => you can set independent IRQ for each port individually enhanced mode => share IRQ, based on IRQ setting of CH#1 3. Control mode: A1 => Auto for RS-422/485 mode B1 => Auto for RS-485 mode, Auto and Always on for RS-422 mode 4. PCB layout: A1 => two-layer board B1 => four-layer board ( enhance noise immunity )