Tuesday, July 19, 2011

The Performance of Tag Name PLC





The MVME2100 CPU with an interface of 100baseT-network communicated to the PLC with its connection of 10baseT through a dual speed hub. The other office PC and a file server of Linux were linked to the same hub. The utilization of network was usually below 2%. On average, 11 milliseconds are required to relocate a single tag, be it a scalar DINT, REAL, or BOOL an array of 352 BOOL or 15 REAL values. Since the driver of EPICS joins multiple requests up to the buffer limit of PLC, around 15 tags, each with a 15-character name, can be interpret in one transfer of around 20 ms while the divide transfers would need more than 160 ms.

In an effort to reproduce a common application, an IOC was constructed with 352 records of binary input, scanning the BOOL array elements at 10Hz, and 120 records of analog input, scanning three 40-element REAL arrays at 2 Hz. The whole array of BOOL transfer was handled in 15ms, all arrays of REAL were transferred in 25ms, so that the records could simply be updated at the selected scan rate. The PLC “System Overhead Time Slice" was set to 10% in these measurements. After growing it to 50% and linking the PLC CPU and to a switch of network, the average times for a single tag are decreased to 7ms.

It is potential to exploit the specification of published EtherNet/IP together with the openly existing Allen- Bradley extensions to interpret and write tags on a system of Control-Logix. The support of EPICS is suitable to use. The configuration of record can be updated at runtime; transfers are combined up automatically to the buffer limit of PLC. There is no need to identify the affected tags as “consumed” or “produced”, nor does it need a network-wide assessment of timing values for arrangement relocates as the original Control-Net did. Because of the nature Ethernet, the accurate transfer time varies. A topology of switched network will facilitate minimize differences. The interest values should be placed in arrays to achieve the required throughput. The intelligible tag names can be utilized to alias the meaningless relocate array elements to maintain the readable of ladder logic. The service codes of published CIP do not permit browsing of type information and PLC tag names, which would permit an even more EPICS driver of user-friendly. Each tag transfer is a roundtrip demand with explicit messaging. The 1756-ENET module of tested version not support embedded messaging.



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