1. Field of the Invention
This invention relates to a programmable controller that is now widely in use for the control in steel, paper manufacturing plants, such public systems as water supply and drainage, industrial systems such as automobile industry.
2. Description of the Related Art
The conventional data communication for exchanging data among programmable controllers (hereinafter, referred to as PC) is described with reference to FIG. 8. When making the data communication between programmable controllers PC1 and PC2, an address was designated so far corresponding to the construction of equipment such as register numbers (REG. NO.) or device numbers stored in a data storage means of the subject programmable controllers PC1 and PC2. That is, the data exchange using actual functional names (logical names of variables) such as limit switch, emergency shutdown switch, etc. that were used in programs was so far not performed.
A defect of a conventional method described above is that it is not able to access specific data unless being always aware of the internal structure/program structure of an object PC. Even when a PC to be subject to the data communication is a PC that executes a similar program, if a program construction and a memory arrangement differ even slightly, it is necessary to change an access method.
Further, if a communicating PC's program is changed when it is under the execution, there is no means to follow the change.
On the other hand, when data are exchanged in a name of variable, if the arrangement of a subject variable in the other controller is changed, a method to correct it automatically will become necessary. When another program is programmed using a name of variable, it is convenient to gain access to a variable by designating that variable name from the viewpoint of the programming but it is troublesome to obtain an actual address of a variable from a demanded variable name and a time is also needed and furthermore, an address of a variable may possibly be changed constantly with the PC program change in the other controller and it becomes necessary to cope with it and therefore, this method is actually not in practical use.