Logix5000 Controllers General Instructions - SLAC Confluence

Special Instructions (FBC, DDT, DTR, PID) 513Relay LadderStructured TextPID_timer.pre := 1000TONR(PID_timer);IF PID_timer.DN THENEND_IF;PID(TIC101,Local:0:I.Ch0Data,Local:0:I.Ch1Data,Local:1:O.Ch0Data,0,Local:1:I.Ch0InHold,Local:1:I.Ch0Data);In this method, the loop update time of the PID instruction should beset equal to the timer preset. As in the case of using a periodic task,you should set the analog input module to produce the processvariable at a significantly faster rate than the loop update time. Youshould only use the timer method of PID execution for loops withloop update times that are at least several times longer than theworst-case execution time for your continuous task.The most accurate way to execute a PID instruction is to use the realtime sampling (RTS) feature of the 1756 analog input modules. Theanalog input module samples its inputs at the real time sampling rateyou configure when you set up the module. When the module’s realtime sample period expires, it updates its inputs and updates a rollingtimestamp (represented by the .RollingTimestamp member of theanalog input data structure) produced by the module.Publication 1756-RM003I-EN-P - January 2007