Thursday, November 25, 2010

Serial Base Devices of PLC-IOC Communications

The sign and requirement systems, market availability of specific types on instruments and collaboration with other SNS vacuum system design group resulted in instrument selection controllers with serial communications as the primary means for configuration, monitoring and control.

Although the target was to install all I/O and control in the PLC, the IOC (Input/Output Controller) was chosen to host serial instruments. Now available, serial ControlLogix modules involve IOC and PLC software development for full functionality. As the serial devices are not needed at the PLC level, the IOC was better to choose.

Driver support and device driver development is required for housing the serial ports in the IOC. Serial device support within EPICS is abundant and more of a quantity that is known.

The control logic within the PLC may require inputs from the serial devices as well as initiate control but loop time requirements are in the seconds order for which network latency would not cause problems.

Loss of PLC-IOC communication requires PLC ladder logic to be designed to handle such an event. Currently serial network layout, specifically RS-485 is in the design stages. The physical locations of performance requirements, device controllers, expected network load per device and related subsystem functions are important considerations when laying out the RS-485 networks.

It may be need to distribute the PID control loop across the PLC and the IOC. Mode change from RF control to temperature control also swaps out the measured variable and PID parameters. Typically standard PID control software does not handle this correctly. The PID software in the IOC does the control software. The configuration under the consideration is for inputs into the PLC, PID in the IOC, data transferred to the IOC, control variables to the PLC, the PLC verifies control values and issues control signals to the final control elements.

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