RELAY LADDER CONTROL SYSTEM FOR SIMULATION AND MONITORING

An improved method of implementing and solving relay ladder logic, such as in programmable logic controllers, more efficiently to allow greater flexibility is provided. Solving relay ladder logic is performed with scan times dependent only upon the number of input and state changes on a scan, and the resulting number of flow changes at ladder connectors. The scan time will therefore be independent of the size of the ladder. The method comprises the steps of recording the structure of a relay ladder and the current flow on the ladder via a ladder coding convention which specifies adjacent relay ladder connections for a particular diagram. Based upon input or state variable changes or events, the process of the invention will act to toggle all of the ladder contacts associated with these events between open and closed positions. This procedure is accomplished by following precomputed lists of such contacts which will be affected by a particular event or change in system model variables. Once the scan events have been posted in the ladder, the scan continues with a processing sweep which computes flow change across one column of the ladder at a time. In each column during a scan, each posted event which has been updated in the control model may be retrieved and expanded across the ladder to vertical paths of the next column to the right in the ladder. When the column sweep reaches a coil element, the changed state variable will be output and retained for the next scan calculation. The relay logic event sweep performs a left to right computation of events or changes in the input or state variables of the control model ladder flow, such that the computation will always move downstream with respect to the power flow on the ladder. This insures that flow change is not doubly calculated on the same scan, returning to an original value. The process maintains a complete record of ladder flow on every scan, and is adaptable to any relay ladder logic system, having no formatting restrictions.