The Source radio buttons on the DL Properties dialog define whether data is read from the OPCServer using Device or Cache reads.
Note: When using Cache reads the values returned will be the most recent values held within the OPCServers Cache. The 'freshness' of this data is governed by the OPCServers max Cache update rate and whether any other clients are performing Device reads for the OPC items that are being scanned.
Tip: To guarantee that the values returned are 'fresh' always use Device reads.