A programmable sequence controller for controlling a machine or process operation including a main low-security programmed sequencer and a high-security programmed sequencer through which selected outputs from the main sequencer must pass before being allowed to control the machine or process operation. This high-security sequencer generates a sequence of steps which relate to machine or process commands under conditions critical to the operation of the machine or process and is controlled in a predetermined order. Any critical output requested by the main sequencer is caused to initiate sequencing by the high-security sequencer and is tested by the high-security sequencer as to the propriety of the generation of a permitted output to the machine or process. The high- security sequencer in this manner effects an improved interlocking between the various critical output commands and actual operative conditions.
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