Monday, November 8, 2010

PLC Control System Integration

The DL405 series PLCs have two different types of base controllers: the Dl405 CPUs and the Ethernet Base Controller (EBC). The DL405 CPU’s provide traditional CPU/Ladder Logic type control of the PLC, whereas the EBC eliminates the CPU, and provides “pass thru” to allow the PC to control the PLC’s I/O directly.

In combination with the DL405 CPU, the Ethernet communications module allows the PC to read and write the PLC’s memory without special communication software in the PLC. This led to the idea of using a “dual port” memory model, where machine status and other parameters are communicated between the PLC and the control PC thru the PLC’s memory. This provides a system with PLC type control, but PC based remote configurability.

Channel information for CAMD’s control system is stored in a PostgreSQL database, and is organized by device type, with device specific parameters as needed. As the CAMAC channel information is organized by crate, slot, channel address, and channel type, this seemed logical approach to organize the PLC channel information. For the EBC type controller, this provided an exact match for the access method that the communications library provides.

For the CPU based controllers, additional abstraction is necessary. The DL405 CPU architecture provides a flexible memory map: inputs, outputs, control registers, and timers can be accessed directly, or as a “V memory” address.

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