Logix5000 Controllers General Instructions - SLAC Confluence

512 Special Instructions (FBC, DDT, DTR, PID)on an unwinder spool, require loop updates as fast as every 10milliseconds or faster.Because the PID instruction uses a time base in its calculation, youneed to synchronize execution of this instruction with sampling of theprocess variable (PV).The easiest way to execute the PID instruction is to put the PIDinstruction in a periodic task. Set the loop update time (.UPD) equalto the periodic task rate and make sure that the PID instruction isexecuted every scan of the periodic taskRelay LadderStructured TextPID(TIC101,Local:0:I.Ch0Data,Local:0:I.Ch1Data,Local:1:O.Ch4Data,0,Local:1:I.Ch4InHold,Local:1:I.Ch4Data);When using a periodic task, make sure that the analog input used forthe process variable is updated to the processor at a rate that issignificantly faster than the rate of the periodic task. Ideally, theprocess variable should be sent to the processor at least five to tentimes faster than the periodic task rate. This minimizes the timedifference between actual samples of the process variable andexecution of the PID loop. For example, if the PID loop is in a 250millisecond periodic task, use a loop update time of 250 milliseconds(.UPD = .25), and configure the analog input module to produce dataat least about every 25 to 50 msecs.Another, somewhat less accurate, method of executing a PIDinstruction is to place the instruction in a continuous task and use atimer done bit to trigger execution of the PID instruction.Publication 1756-RM003I-EN-P - January 2007