J1708 Half Duplex Collision Detection
SAE J1708 uses RS-485 transceivers, but connects the serial transmit data to the enable line of the driver rather than to the data line. This means that the driver is effectively switching directions on every bit. This is similar to CANbus, in which one of the bit values is "dominant" and the other is "recessive".

The logic of each node is supposed monitor the recessive bits of the MID byte to determine whether any other node is transmitting a dominant bit at that time. If it detects this condition, the other node has a higher-priority message, and this node should immediately drop out and retry its message later.