control structure - PLC Programming & Automation

FBC and DDT: Comparing an Array Against a Reference, and Which One Reports What

FBC and DDT operand blocks with Source InputState element 0, Reference GoodState element 0, Result Mismatch element 0, compare control CmpCtrl with Length 128, and result control ResCtrl with Length 32

One sentence in 1756-RM018A separates these two instructions: each time the DDT instruction finds a mismatch, the DDT instruction changes the reference bit to match the source bit, and the FBC instruction does not. Everything else about them — the operands, the two control structures, the result array, the fault codes, the 0.16 µs per … Read more

SQO and SQI on a ControlLogix: the Sequencer That Will Not Step

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A nine-element DINT array drawn as the sequencer table for an eight-station indexer, each element showing its binary step pattern and what the step does, with element 0 marked as the park position, the position pointer at element 3, and a wrap arrow back to element 1

SQO and SQI on a ControlLogix: the Sequencer That Will Not Step SeqCtl.POS sat at 1 for three days on a 1756-L83E while the indexer kept running on step one, and the rung holding the SQO had nothing in it at all — left rail, instruction, right rail. That is the entire fault, and the … Read more

FAL and FSC Over an Array: the Control Tag, the Position and the Scan It Takes

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A table comparing what each CONTROL structure member means to a FAL and to an FSC: EN, DN, ER, LEN and POS shared, with IN and FD highlighted as FSC-only, plus a boxed note that Length and Position are pseudo-operands written at download

FAL and FSC Over an Array: the Control Tag, the Position and the Scan It Takes An FSC with a Length of 10 found its mismatch at element 3 on a 1756-L83E and then stopped looking. .FD was 1, .POS was 3, .IN was 1, and elements 4 through 9 were never compared — not … Read more