Because of the effect on the segment's bandwidth, it is recommended that you limit write requests to fieldbus function blocks to three (no more than 30 outstanding requests per controller for any one time) and use write requests only when necessary. Like module scan rates, write requests can impact the rate at which the View List is scanned and can use up a good deal of the fieldbus bandwidth. For example, if a Calc block's output is linked to an external reference that is tied to the SP of a fieldbus PID block, the system will attempt a write of the value (over the fieldbus) to the fieldbus device with each execution of the module.
Be especially careful when using periodic writes to static fieldbus parameters in an expression since this type of write can increment the block's static revision parameter (ST_REV), which then causes the controller to issue two more requests to read static View List data. Refer to the FOUNDATION Fieldbus specification for more information on the View List. For the DeltaV system, VIEW_3 is constantly being read, but VIEW_4 is requested when the static revision counter (contained in VIEW_3) is incremented.