The PCB attaches to the back of the OP-320A unit, which is a cheap operator console panel you can get off aliexpress or ebay for about $50, you remove the PCB with the unit and attach this as a replacement to give arduino compatibility.
Wiring mostly copied from teensy++ and uno so something of a mix here, Uses a standard Arduino shield header but you will definitely need to adjust your sketches as some pins will be dedicated to the LCD and keypad. Also added two headers for the 4-pin relay boards and RS-232 boards that cost a dollar or two off ebay. All pins labelled for ease of use.
Note in my system I did not use the back plastic cover as it was going into another panel and the original back cover wont we deep enough to accommodate the unit so you will have to make a back housing and it will be deeper than the original OP-320A.
Second PCB seems to work OK with the bootloader supplied on the chip, the third version (here) has minor layout changes and should be 100% good.