Monday, July 18, 2011

The Implementation of EPICS Support

Unrelated messaging is employed since the benefits of related messaging do not relocate from Control-Net to EtherNet/IP: Ethernet doesn’t keep bandwidth; guaranteed delivery is already settled by TCP. The comparison of connected to unconnected messaging is effected a little smaller messages and a 3% enlarge in throughput for requests of CIP_Read_Data. Since a drawback, the application of client must send requests at the recognized update time of the connection or faster. The delays of temporary Ethernet cause the PLC to close the link. The Multi-Request Service of ControlLogix is utilized to join the requests of CIP_Read/Write_Data till either the total response size expected achieves the buffer limit of PLC about 500 bytes.

For each PLC, the driver code of vxWorks places the tags in scan lists according to the requested update rate. One thread per PLC switches all requests of read/write. The device support of EPICS permits analog, binary and multi-bit records to apply the driver for input and output. Tags must refer to a single array element or a structure element, a scalar value, not whole arrays or structures. The data types of PLC BOOL, SINT, INT, DINT and REAL are settled. One can change the configuration record at runtime, without rebooting the IOC, such as the tag name that a record submits to can be put back. The driver cuts off from the PLC and effort interrupted reconnects in case of a communication timeout or error.

The driver joins demands for elements of array into one array transfer from the first to the highest demanded element per default. This guides to an important decrease in transfer times, but may have side consequences: The IOC will forever write the entire array whenever more than one part has been changed by the records of output. If the same PLC array has been customized by another source (display of PanelView) while the last transfer, the IOC is unaware of these updates and will overwrite them. An array relocate is also size-limited by the abovementioned buffer limit PLC. The configuration of record permits divide array element relocates as a workaround for these cases.

The driver transfers a message of CIP_Write_Data whenever the record is method for output records. Otherwise it will occasionally interpret the tag from the PLC and keep posted the record of output if the PLC value differs from the other in the record. The driver remains the information of statistical such as error counts, last/minimum/maximum transfer time for every scan list. The records of analog input permit access to these costs.


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