1. Field of the Invention
The invention relates generally to digital controllers for controlling industrial machines and processes, and more particularly, to programmable controllers.
2. Description of the Prior Art
In the simplest system architecture for a programmable controller, the controller includes a processor module and several I/O modules which are supported in the same equipment rack and electrically connected through a backplane circuit board. The backplane circuit board or simply "backplane", as it is often referred to, is a circuit board with numerous printed circuit paths or traces extending horizontally across the back of the rack to interconnect the modules in the rack.
In a more complex system architecture shown and described in Schultz et al., U.S. Pat. No. 4,413,319, issued November 1, 1983, a programmable controller can also include a stand-alone main processor and a number of "remote" I/O racks strung out along serial communication channels. A scanner module interfaces the main processor to the serial communication channels. The scanner module in the Schultz et al. patent transmitted blocks of I/O status data to adapter I/O modules residing in remote I/O racks. However, the Schultz et al. system was limited to control by a single system processor and one of the adapter modules could not be placed in the same rack with an in-rack processor module.
As technology moves toward greater automation of manufacturing operations, it has become desirable to connect programmable controller systems in a system hierarchy, in which a large-capacity supervisory processor exercises some control over, and monitors, functions for a group of local area programmable controllers, each having its own local area processor. Such a local area programmable controller might take the form of the small programmable controller shown and described in Struger et al., U.S. Pat. No. 4,293,924, issued October 8, 1981, or it might take the form of the multi-rack controller of Schultz et al. mentioned above.