I have discovered a few problems with the MDM board. 1. Missing pull up on tx out pin on the rs485 driver. I discovered that when the ~re (receive enable) pin on the 485 driver goes high, the tx out pin goes low. This high to low transition will appear as a start bit and cause a character interrupt on the xmega. By pulling up th tx out pin the line will stay high when the receiver is turned off. 2. The chip select lines for all six motor drivers are connected together but are not connected to the xmega.