Principle
When accessing variables over S7 connections, the OPC Server for SIMATIC NET provides an optimization algorithm to increase the data transmission speed.
Several simultaneous jobs to access individual variables are converted internally to create a single access to an array on the communication partner. This reduces the number of data packets transported over the network. Utilization of single packets is improved and the payload of a packet is increased.
This optimization is used for both read and write access and is enabled as the default setting. For the OPC client, for example an OPC-compliant HMI device, this optimization algorithm is invisible.
In order to be able to ensure the read or write processing of the S7OPT data within a CPU cycle, you must set the value for the parameter "Buffer size for optimization" in the OPC properties of the S7 connection less than or equal to 64 bytes for an S7-1200 CPU (firmware version V4.0 or higher) or less than or equal to 512 bytes for an S7-1500 CPU (firmware version V2.0 or higher). For an S7-1500 CPU where the firmware version is less than V2.0, you must set the parameter less than or equal to 64 bytes.
Further information
Additional details are contained in the documentation for the OPC server:
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Industrial communication with PG/PC Volume 1
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Industrial communication with PG/PC Volume 2