AN EVENT CHAINING RELAY LADDER LOGIC SOLVER

Relay ladder logic (RLL) remains a dominant modeling language in progr ammable logic control (PLC) systems. RLL uses now over a relay ladder structure as the control model state. Event chaining, a requirement fo r discrete systems, is applied to ladder solving. An event chaining re lay ladder solver, the ''Event Sweep'', is described in detail. The Ev ent Sweep maintains a complete ladder flow state, and therefore suppor ts on-line display of ladder logic. Scan times of existing ladder solv ing methods depend on the size of the ladder model. It is shown how RL L 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 ch anges in ladder flow caused by the opening and closing of contacts.