Patent ID: 7080137

Claim:
A communication system between a programmable logic controller equipped with an HTTP server and client machine via a network, said system comprising: group sheets in XML format including group elements or signals for watching or controlling a process automated by an application of the programmable logic controller; means for reading from the programmable logic controller said elements or signals; means for writing said elements or signals from the programmable logic controller on said group sheets; means for reading said group sheets and for writing on said group sheets upon a query from the HTTP server, wherein the means for reading the group sheets and for writing on the group sheets includes means for generating a replicated sheet grounding the element modifications of a group sheet, said modifications taking place between two successive server pollings; means for storing and creating write sheets in XML format, allowing said means for writing to change said elements or signals on said group sheets upon a query from the HTTP server; and means for writing sheets in a controller cycle so as to update the sheets and associated elements, wherein the programmable logic controller is configured to operate by cycles and further comprises a mechanism based on indicators signaling that said elements or signals have been changed by said means for writing on said group sheets upon a query from the HTTP server, thereby enabling the programmable logic controller to test whether a value of an element or signal has changed since a previous controller cycle.