PLC Variables

The PLC variables act as the interface between the local process image of the bus controller and the fieldbus process image for a higher-level controller. This will first be explained on the basis of the default setting.

Bus Terminals in the BC's local process image

All the connected Bus Terminals are assigned to the local process image. The PLC variables are located from address 128 onwards. You can change both this starting address and the length of these data (default 16 bytes).

Data that is to be read by a higher level controller is written into the output process image. This is input data for the higher level controller. Data that is to be transferred from the higher level controller to the BC is output data for the controller and input data for the BC. The following diagram may clarify this.

Assign bus terminals to the higher level controller

You can also assign the Bus Terminals directly to the higher level controller. The general scheme of the fieldbus process image is such that the analog Bus Terminals are mapped into this process image first. The digital Bus Terminals then follow, while the PLC variables come last.

Remember that none of the process images in use may be larger than 512 bytes.