Ladder Tutorial

Posted on Feb 5, 2014

In this tutorial, I will show you how to write a very simple program. I am assuming that you have written ladder logic before, and that you have some basic familiarity with microcontrollers, but that you have never used LDmicro. If you don`t know very much about ladder logic or PLCs, then the plcs. net tutorial might be helpful to you. Our device w

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ill have one pushbutton, and one LED. At startup, the LED will be off. When you press the pushbutton once, the LED will turn steady on. The second time you press the pushbutton, the LED will start blinking. The third time that you press the button, the LED will turn off again. On subsequent presses, the cycle will repeat. We will be using a PIC16F876, which is easily available from Digikey or other online distributors. It comes in a number of different packages; I chose a DIP. Note that as of Nov 2009, the PIC16F876 is no longer recommended for new design. This means that it will probably get discontinued at some point in the next few years. You may prefer to instead use a PIC16F886, which is pin-compatible. If you do, then make sure to specify the correct part when you compile, since the `F886 is not code-compatible. The microcontroller (IC1) is part number PIC16F876-20I/SP-ND at Digikey. Almost any three-terminal resonator (U1) will do; you might try a 535-9356-ND or an X909-ND. The only thing that might confuse you is that the pushbutton goes to Vdd, and there is a pull-down. You might be more used to seeing a pushbutton to ground with a pull-up. For TTL, this mattered. For modern CMOS it does not, and I find this `active HIGH` arrangement less confusing than the traditional `active LOW` circuit. Also, I chose to use a ceramic resonator with internal capacitors, U1, instead of a crystal and two ~20pF caps. A crystal...

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