Programmable controller with fault detection

Claim 1

A programmable controller connectable to an external unit, comprising: a central processing unit for providing a first signal; an output memory, coupled to said central processing unit, for storing the first signal from said central processing unit and for providing, as an output, a second signal, the second signal also being fed back to said central processing unit; an output circuit, coupled to said central processing unit and said output memory, and connectable to the external unit, for receiving the second signal from said output memory and for providing, as an output to the external unit, a third signal, said output circuit including means for feeding back the third signal to said central processing unit, said output circuit and said output memory being susceptible to damage and abnormal operation when said output circuit is connected to the external unit, said central processing unit including: means for comparing the first and third signals, for comparing the first and second signals and for generating a first abnormality signal indicative of an abnormality in said output circuit if the first and third signals do not coincide and for generating a second abnormality signal indicative of an abnormality in said output memory if the first and second signals do not coincide; and a display device, coupled to said central processing unit, for providing a display identifying an abnormality in one of said output circuit and said output memory when the corresponding one of the first and second abnormality signals is generated.