Abstract
Relay ladder logic (RLL) remains a dominant modeling language in programmable logic control (PLC) systems. RLL uses flow over a relay ladder structure as the control model state. Event chaining, a requirement for discrete systems, is applied to ladder solving. An event chaining relay ladder solver, the “Event Sweep”, is described in detail. The Event Sweep maintains a complete ladder flow state, and therefore supports on-line display of ladder logic. Scan times of existing ladder solving methods depend on the size of the ladder model. It is shown how RLL solving can be limited to flow calculation along “hot” flow paths, and to “eligible” networks in which contact switches can influence coil values. The Event Sweep algorithm limits solving to downstream changes in ladder flow caused by the opening and closing of contacts.
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