You were right. All I had to do to get it working was to move the TXEN pin. It works great now. I got the Wiegand to Modbus protocol converter working fine on a single Arduino. Then I enhanced it by writing the door status and controlled the beeper on the RFID reader. So its job done for that project. I did find a way of using the shield by removing the jumper and putting a link over to a spare GPIO pin.