Communication system between a programmable logic controller server and a client machine

An interface between a programmable logic controller and a HTTP server, said programmable logic controller comprising an application that automates a process, said interface comprising:

• group sheets in XML format including group elements or signals for monitoring or controlling said process automated by said application of said programmable logic controller, said elements or signals storing values of controller variables;
• means for reading said elements or signals on said group sheets from the programmable logic controller;
• means for writing a controller variable into said elements or signals on said group sheets;
• means for reading said group sheets and for writing on said group sheets upon a query by a client machine on the HTTP server, wherein the means for reading the group sheets and for writing on the ground sheets includes means for generating a replicated sheet grouping 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 by said client machine on the HTTP server, thereby enabling the client machine to control the programmable logic controller, the programmable logic controller further configured to test whether a value of an element or signal has changed since a previous controller cycle.