s2 or s3 are open the corresponding signal at burner controller 14 is a logic low signal fig2 is an enlarged portion of heating system 10 shown in fig1 input protection circuit 24 is shown coupled to switch s1 input protection circuit 24 is typically coupled to each input to burner controller 14 to protect burner controller 14 from being damaged by voltage spikes caused by noise or static discharge input protection circuit 24 is either implemented externally to burner controller 14 or internally included in input protection circuit 24 are resistors r1 and r2 and diodes d1 and d2 fig2 also shows switch terminals 26 and 27 and input terminal 28 fig3 is a plot of a control signal which typically appears at switches s1 s2 and s3 in this embodiment fig3 shows a plot of the signal appearing at switch terminal 26 shown in fig2 when switch s1 is closed control signal cs shown in fig3 is applied to input protection circuit 24 fig4 shows the signal appearing at input terminal 28 of burner controller 14 when switch s1 is closed the control signal cs appearing at input terminal 28 is substantially a square wave during normal operation burner controller 14 samples the signal appearing at input terminal 28 once each half cycle by doing this burner controller 14 determines the state of corresponding switch s1 and hence the state of the sensed control parameter for example burner controller 14 samples the signal appearing at input terminal 28 at approximately the midpoint of time period t1 to verify that control signal cs is above a threshold voltage v t then at approximately the midpoint of time period t2 burner controller 14 again samples control signal cs to verify that.