3-11
3-3 Accessing Process Data
(4) Update and data change notification (subscription)
SX_MDLW updates the cache area in the OPC server at each update cycle (UpdateRate) of the group. When connection
with IConnectionPointContainer has been established between the OPC client and server, SX_MDLW checks for data
changes at update time and, for an item with a data change if any, automatically transmits the data to
IOPCDataCallback::OnDataChange of the OPC client. Because this data transmission is indicated with specific value “0”
as a transaction ID, the OPC client can recognize it from refresh data.
(5) Quantitative restriction
The number of items that can be handled by SX_MDLW and that of projects accessed when acquiring an address in
D300win are determined by OPC file setup of SX_MDLW. Specifying large parameter values requires a lot of resources,
lowering the performance; therefore, tuning by the system is necessary. See Section 4-1 “Setting SX_MDLW” for the
setting procedure.
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