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Diagnostic circuit for programmable logic safety control systems

  • US 4,415,884 A
  • Filed: 10/07/1981
  • Issued: 11/15/1983
  • Est. Priority Date: 10/07/1981
  • Status: Expired due to Fees
First Claim
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1. In a programmable controller based logic 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 field device, a method of operating a programmable controller based logic system comprising:

  • a. monitoring said first image register, said second image register and the integrity of the series combintion 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 steps (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. and 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.

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