Monday, July 25, 2011

Peripheral Blocks Development of STEP7 PLC

CODAC Interface
The interface of CODAC is based on raw communication socket between the PSH and the PLC from a protocol point of view. This communication is executed in the PLC throughout the blocks of “Open Communications IE”. This family of block is offered only on CPUs which have an Interface of Ethernet embedded. This option is based on a communications assessment of possibilities for Siemens STEP 7 PLCs. This interface will support 4 information types:
• Simple commands
• Collaborative data
• State variables
• Configuration variables
Every data type will be transferred in one DB which has 8 kBytes maximum size. Configuration and states will use the same connexion of TCP, simple instructions and data collaborative will have their own connections of TCP.

Every DB will be developed with UDTs. Every UDT will stand for the type of interface such as states, configuration, commands, of one control function for the plant system FBSs. The block organization and dependencies for the function control of “CWSDHLT-WFC” in the System of Cooling Water.

Generation procedure
The different components of STEP 7 explained previously will be incorporated in source files automatically produced by the SDD. SDD is a toolkit of CODAC used to explain the interface with a PLC and the files of STEP 7 and PSH needed to build this interface are produced automatically. The toolkit of SDD produces a set of plant system specific files of symbol table (*.sdf) and a file of STL (*.awl). These files apply the data blocks of CodacStates, CodacConfiguration and CodacCommands and initialize the data block of CodacChannels. The editor of SDD produced files for the System of Cooling Water. The use of SDD is illustrated after the SPSS is built, the file of symbol table (*.sdf) and the file of STL (*.awl) should be imported to the project of STEP 7 as explained below.
1. Open the table symbol and import the system of plant specific file symbol table (*.sdf) and save.
2. Insert external source from the system of plant specific file SDD generated ‘*.AWL’ in the folder of CPU | S7 Program | Sources and compile.

System Health Monitoring
The health monitoring implementation is still TBD. In sequence to avoid breaks down of the PLC, the below OBs will be loaded in the PLC:
• OB122 : access interrupt of I/O error.
• OB121 : error interrupt of programming.
• OB86 : failure interrupt of IO-Device.
• OB82 : interrupt of diagnostic.


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