Chapter 4. Ladder Diagram (LD) ProgrammingGFK-2950C February 2018 874.3.4 Transition CoilsPACSystems controllers provide four transition coils: PTCOIL, NTCOIL, POSCOIL, and NEGCOIL.POSCOIL and NEGCOIL are updated every time they are called.PTCOIL and NTCOIL are updated once per CPU scan.For examples showing the differences in the operation of the two types of transition coils, seeExamples Comparing PTCOIL and POSCOILPOSCOIL and NEGCOILWarning■ These transition coil instructions should not be used ina parameterized block or user-defined function block(UDFB) with a parameter or member. In these cases, anR_TRIG or F_TRIG should be used instead.■ Do not override a transition coil by putting a force onits reference bit. If a transition coil is overridden, thecoil has no effect on the bit, and if the override is thenremoved, the coil might be set ON for one sweep. . Thiscan cause unexpected behavior in the Controller logicand in field devices attached to the Controller.■ Do not write to the reference bit of a transition coilusing any other instruction or from an external device.Doing so will destroy the coil’s one-shot nature and thecoil may not behave as described.■ Do not use a transition contact with the samereference address used on a transition coil because thevalue of the transition bit, which stores the power flowvalue into the coil, will be affected.Positive Transition Coil (POSCOIL) Negative Transition Coil (NEGCOIL)If:■ the transition bit is OFF, and■ the input power flow is ON,the POSCOIL sets the reference bit of itsassociated variable ON until the coil is executedagain. When the coil is executed again, it sets itsreference bit OFF.Note: When the Positive Transition Coil sets itsreference bit ON, it also sets its transitionbit to ON. The next time the PositiveTransition coil executes, it finds itstransition bit set to ON and sets itsreference bit to OFF.If:■ the transition bit is OFF, and■ the input power flow input is OFF,the NEGCOIL sets the reference bit of itsassociated variable ON until the coil is executedagain. When the coil is executed again, it sets itsreference bit OFF.Note: When the Negative Transition Coil sets itsreference bit ON, it also sets its transitionbit to ON. The next time the NegativeTransition Coil executes, it finds thetransition bit set to ON and sets itsreference bit to OFF.
