As mentioned earlier, combinational logic produces results after a delay. Sequential circuitry is what adds memory to our logical systems. Here are truth tables describing the six main elements:Ĭombinational logic is great, but without adding sequential circuitry, modern computing would not be possible. Truth tables are simple plots which explain the output of a circuit in terms of the possible inputs to that circuit. The descriptions above are adequate to describe the functionality of single blocks, but there is a more useful tool available: the truth table. One output may be connected to multiple inputs, however. While inputs can be connected together, outputs should never be connected to one another, only to other inputs. They will, however, only have one output.ĭigital logic circuits are usually represented using these six symbols inputs are on the left and outputs are to the right. Two input devices are standard, but you will occasionally see devices with more than two inputs.The A-B-Q type terminal notation is standard, although logic diagrams will usually omit them for signals which are not inputs or outputs to the system as a whole.Usually, the name of the gate is not printed the symbol is assumed to be sufficient for identification.The output of an inverter is a 1 if the input is a 0, and vise versa.Ī few things of note about the above image: Inverters aren't truly gates, as they do not make any decisions. There is a sixth element in digital logic, the inverter (sometimes called a NOT gate). NOR gate - output is 1 if BOTH inputs are 0.NAND gate - output is 1 if AT LEAST one input is 0.XOR gate - output is 1 if ONLY one input is 1.

OR gate - output is 1 if AT LEAST one input is 1.

AND gate - output is 1 if BOTH inputs are 1.

There is a small amount of discussion on boolean logic in there, but we'll be going much further into the topic here.Here are some other topics you should be familiar with before getting started.Ĭombinational circuits are built of five basic logic gates: Suggested Readingīefore getting started, it might be a good idea to review our tutorial on binary numbers, if you haven't already. There are also some subtleties in programming that are important to understand we'll get into that once we've covered the basics. Understanding digital logic makes complex decision making possible in programs. Programmingĭigital logic is important in programming, as well. Typically, a sequential circuit will be built up of blocks of combinational logic separated by memory elements that are activated by a clock signal. Sequential circuits have a clock signal, and changes propagate through stages of the circuit on edges of the clock. Combinational logic changes 'instantly'- the output of the circuit responds as soon as the input changes (with some delay, of course, since the propagation of the signal through the circuit elements takes a little time). Digital circuitryĭigital logic circuits can be broken down into two subcategories- combinational and sequential.

Put simply, it's the system of rules that allow us to make extremely complicated decisions based on relatively simple 'yes/no' questions. Digital, or boolean, logic is the fundamental concept underpinning all modern computer systems.

Sequential logic diagram