From the course: Learning PLC Ladder Logic: 1 The Basics

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Timer status bits (EN, DN, and TT)

Timer status bits (EN, DN, and TT) - Ladder Logic Tutorial

From the course: Learning PLC Ladder Logic: 1 The Basics

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Timer status bits (EN, DN, and TT)

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- [Instructor] A timer instruction comes with three main status bits. Timer done bits, timer timing bits, and timer enable bits. And depending on the PLC type, the bits are either attached to the timer instruction or hidden. A timer done bit is true when the accumulated value is equal to the preset value. It is basically on when the timer is done timing. It changes from false to true or from true to false depending on the type of the timer instructions you use. The bit is commonly used to control another logic when the timer is done timing. A simple example in my process that the heating of the liquid is to begin 30 seconds after signal is received from a limit switch. So we use a timer on delay to set a 30 second and use the done bit to start the heating process. So here, we use an XIC instruction for the limit switch and when the limit switch is activated, the timer starts timing for 30 seconds. For the timer instructions, for simplicity, let's use a timer base of one, second and…

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