generally indicated at 20 for detecting ionic current in the ignition system after combustion of fuel in the engine finally fig1 shows a block diagram of detection logic 22 with various vehicle inputs for providing a misfire output signal there is only one set of detection logic 22 for the vehicle not one per cylinder also more than one coil spark plug combination can be connected to the input of the circuit 20 at node 24 it has been found that two coils per circuit 20 is optimum to keep signals from encroaching upon the time slices reserved for others this phenomenon becomes prevalent at high rpm three signals from the vehicle are required by the detection logic 22 these are 1 ignition diagnostic monitor idm the idm occurs synchronously with the spark event one positive pulse per firing event is used to identify the start of the ignition discharge the idm pulse for cylinder 1 has a different pulse width so that cylinder identification and synchronization can be achieved 2 clean tack output cto one negative pulse per cylinder event negative edge occurs 9 crank degrees before top dead center fig2 shows the timing relationships of the cto and idm signals previously described the position of the idm signal is typically prior to the cto falling edge but can also follow this edge fig2 also shows the detailed relationship between cto idm and.