This chapter focuses on the issues of specification in direct digital control (DDC). The specification is the document that sdefines for the contractor what is required. It is also the document that is used to check that the contractor has completed its contract and that performance is being met. To effectively achieve these objectives the specification must be clear and unambiguous to write a specification that is clear and unambiguous you need to know your objectives. Establishing the objectives is always dependent on what the client wants and is willing to pay for in both the initial construction and also in the ongoing operation and maintenance. One of the most useful features of DDC systems is the ability to monitor what is happening in the system. Seeing the system working and checking on performance is greatly assisted by recording, or trending, values of points at set intervals. For checking the performance of a control loop a short time interval, say 1 s, may be used. The minimum time interval and number of records will typically be determined by the controller and its memory capacity.
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