Diagnostic circuit for programmable logic safety control systems

In a programmable controller based burner management system having an alarm system for alarming malfunctions, an alternating current voltage source, first and second image registers, electronic input and output modules, said output modules containing solid state switches capable of switching the output module load power, and first and second output module solid state switches connected in series to energize and deenergize an energize-to-start, deenergize-to-stop burner associated field device, a method of operating a programmable controller based burner management system comprising:

• a. monitoring said first image register, said second image register and the integrity of the series combination of said first and second solid state switches;
• b. independently of and simultaneously with step (a), monitoring said first image register and the integrity of said first solid state switch;
• c. independently of step (a) and (b), monitoring intermittently said second image register and the integrity of said second solid state switch;
• and
• d. alarming an unsafe condition whenever said monitoring indicates the failure of said first image register, said second image register, said first solid state switch, said second solid state switch or the series combination of said first solid state switch and said second solid state switch.