state or a failed action state and a speed down fast state or an indeterminate state the speed up state has a 4 bit complement of the no action state and the speed down state has a 4 bit complement of the reserved state in this embodiment the speed up fast state has a 4 bit complement of the pto on state and the speed down fast state has a 4 bit complement of the pto off state the remaining four bit codes that have not been assigned a function state are each a failure state these coding constraints also provide communication failure modes that are generally desirable from an engine management standpoint in both tables 1 and 2 a transition from one function state to another function state requires a two bit error table 3 shows an exemplary embodiment of the four bit coding shown in table 1 with an internal cab switch stationary or mobile providing three state inputs to the pto control module table 4 shows an exemplary embodiment of the four bit coding shown in table 1 with a stationary external cab switch providing three state inputs to the pto control module table 5 shows an exemplary embodiment of the four bit coding shown in table 2 with an internal cab switch stationary or mobile providing three state inputs to the pto control module table 6 shows an exemplary embodiment of the four bit coding shown in table 2 with a stationary external cab switch providing three state inputs to the pto control module in tables 3 6 u201c 1 u201d is a high.