Sunday, July 24, 2011

Input/Output Wrapper of PLC

The aim of this layer is to straight apply the addressing area of Siemens data block at the lowest level. This has 2 benefits:
• It is probable to put the information in subsystems and systems hierarchically.
• All the variables can be done with just one easy block.
Complicated interfaces like positioning modules of FM453 and CP441 serial communications modules will also be applied in this layer. There is a connection between the health monitoring function and this layer. All the variables distributed by the wrapper will be transferred to the system of health monitoring. The system of health monitoring transfers these variables to the interface of CODAC. The point is to have raw values of CODAC accessible on a system screen for debugging purposes.

Interface Switch
Linking a process simulator to the controller provides the following possibilities:
• The software validation without being associated to the process
• Modification of the software and be capable to test these adaptations on a different platform, before loading on the real control unit during incorporation and commissioning.
The switch of interface purely switches the signal variables origin between the simulator and real process. The interface switch control will be a configuration variable of CODAC. This command must have some security connected with it in the sense that it cannot be utilized during process.

Engineering Limits
It is required to set limits reflecting the actuator limit or the physical process articulated in engineering format for numerical outputs. The output of PLC may be incorrect if these limits are went over. The limits will be set by variables of configuration.

FBS Wrapper
This block just transmits the variables of signal presented in a naming convention of PBS to a naming convention of FBS. Numerical and digital signal variables are separated because the layers above are different.
Conversion will be necessary for the majority of the numerical signal variables. This conversion can be quadratic, linear, or of superior orders. All the parameters of conversion will be provided by configuration variables of CODAC.

The idea is to homogenize the core application of PLC code as much as possible. The same devices type should be controlled with the same function of PLC. In fact, the same devices type will be wired with different logic. For instance, a valve: some valves limit switches will be wired with positive logic, 24VDC – position reached, and some with negative logic, 0VDC – position reached, even as control of the valve is similar. The standardization block function would be to process the reversal necessary for all discrete signals, in order to show a standard signal interface for the different devices types to the core application of PLC.

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