So far with our logic gates we've covered almost all possible combinations except for one shown by the truth table below. The ladder logic is very different from the regular OR gate.īut wait! Don't order yet. It's symbol and corresponding ladder logic are shown below. The truth table below shows that it is simply an inverted output of the OR gate.Īgain a little circle is placed at the end of an OR gate to signify the NOR function. Putting the NOT and OR gates together forms.
Basic ladder logic program series#
Now pay close attention to the ladder logic because the contacts are in parallel and not in series like the AND function. The truth table below shows that it is simply an inverted output of the AND gate.Ī little circle (or if you like, a bubble) at the end of a AND gate is used to signify the NAND function. Putting the NOT and AND gates together forms the NAND gate. So the engineering gods decided to make some symbols for these combinations. It's very common to use it in combination with AND and OR. The NOT gate might not look like much help if you haven't programmed much but you'll find yourself actually using it frequently. The ladder logic equivalent for an OR function looks like two normal contacts on top of each other. An easy way to remember this is OR works like addition. The truth table below shows that the output is turned on (1) when any of the inputs are true (1). Last but not least the OR gate is associated with the following symbol that also can have any number of inputs but only one output.
The ladder logic equivalent for an AND function looks like two normal contacts side by side. An easy way to remember this is AND works like multiplication. The truth table below shows that the output is only turned on when all the inputs are true (1). The AND gate is associated with the following symbol that can have any number of inputs but only one output. The ladder logic equivalent for a NOT function looks like a normal contact but with a slash through it. Shown below is a truth table (it doesn't lie) showing all possible inputs and the resulting logical output. So an input of 1 will come out as a 0 and visa versa. It's sole function in life is to invert of flip the logic state. The simplest of all logic functions is the NOT gate. These functions are also called gates as they act like gate keepers for different logic. A series of graphical objects have been used for years to represent these logic elements and they can be easily converted to a common ladder logic equivalent. It would be nice to program like this but computers like to be a little bit more structured.
Basic ladder logic program full#
If the tank is full OR the button is pressed AND there are no alarms then start the process.Relying on a signal to turn on when a wire has fallen off long ago may cause an awkward moment when we truly have to stop the machine in an emergency.) In either case we want to check this for safety reasons.
(This is an example of a failsafe operation as the emergency stop button could be pressed or the wire has been disconnected.
In the PLC it all boils down to our now familiar binary system of a 1 or a 0. In the world of automation these types of TRUE or FALSE conditions come down to a device being ON or OFF, CLOSED or OPEN, PRESENT or ABSENT, 24 VOLTS or 0 VOLTS. Now we could get more complex but all that we'd be doing is using these simple building blocks. You'll notice that they all involve three types of comparisions: AND, OR and NOT. Now these are pretty simple decisions especially if you're a ten year old boy.
If Mom comes out AND orders me inside OR it get's dark then I'll stop playing.It's 6 o'clock AND I'm NOT hungry therefore I'm going to keep playing.If Tommy OR Bob want to play basketball then I'll play too.All though I've always found computers to be quite a bit more logical then human beings.įor an example of how we use logic in everyday life consider these statements: We gather information (input) and based on that we make choices that determine our output. What is true for us is also true of PLCs.
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