Investigate possible problems with reversed polarity RS485 connections

[jcoffland]

The Buildbotics controller will automatically flip the polarity of the RS484 connection if communication is not successful. In one polarity, the line will be in a continual low state which looks like a serial start bit. This may cause repeated zero bytes with frame errors to appear on the AVR serial port. If the VFD starts communication it's first byte (or even several bytes) may be corrupted. This may be causing some VFDs to fail to communicate when the wires are connected in the wrong direction.