Method and system for communicating data between independent controllers

A data communication system for communicating data between a first control unit for controlling a first operation and a second control unit for controlling a second operation, the first and second control units being provided in an image forming apparatus, the first control unit having a first memory for storing a first operative data necessary to execute its own control independently of the second control unit, the second control unit having a second memory for storing a second operative data necessary to execute its own control independently of the first control unit, the data communication system comprising:

• address data means provided in the first control unit for storing address data for each of the items constituting the second operative data stored in the second memory;
• item data generating means provided in the first control unit for generating new data for a selected item of the second operative data;
• transmission data generating means connected with the address data means and the item data generating means for combining the new item data and the address data corresponding to the selected item to generate transmission data;
• data transmitting means connected with the transmission data generating means for transmitting the transmission data to the second control unit;
• data receiving means provided in the second control unit for receiving the transmission data;
• writing means connected with the data receiving means for writing the new item data on the second memory at the address specified in the transmission data;
• the second control unit being further provided with input and output port means having an output bit selectively driven by a bit of one of said new item data at a specified address of the second memory and in direct connection with a specified one of a plurality of actuators for executing the second operation;
• the first control unit being adapted for controlling an input and output operation of information necessary for image forming, the second control unit being adapted for controlling an operation of reading a document image and forming an image on a copy item.