fieldbus devices allow a limited number of links between their function block parameters and parameters in other fieldbus devices. Some devices support input and output links called publisher and subscriber VCRs and other devices support links called Free VCRs. A subscriber VCR is an output from a fieldbus device to an input in another device on the segment. The input device can be another fieldbus device or a controller. A publisher VCR is an output from a DeltaV controller to the input of a parameter in a fieldbus device. For example, suppose a device allowed only four links as inputs to their function blocks and only four links as outputs from their function blocks. This limit is quickly reached if Feed Forward, Cascade, and Track control methods are used because these types of control require more inputs and outputs. During application configuration, the DeltaV system notifies you if the subscriber or publisher limit for a device has been reached.
Free VCRs function as either input or output links or device alarms. Device alarms require one Free VCR. For example, if a device supports five Free VCRs and one Free VCR is used for a device alarm, then four Free VCRs are available to the device.