Two kinds of circuit
Combinational versus sequential
A combinational circuit produces outputs that depend only on its current inputs, while a sequential circuit's outputs also depend on stored history, and distinguishing them tells you whether the circuit needs memory and a clock.
Every circuit you build falls into one of two families, and knowing which you are looking at changes how you reason about it and test it. This bridge names the two families and gives the one-question test that sorts any circuit into the right one, right before you build your first memory cell, the SR latch.
So far you have built combinational circuits without hearing the word. A full adder, a mux, a decoder: give it inputs, it produces outputs, and if the inputs are the same the output is always the same. There is no notion of "before". These circuits are pure functions of their present inputs.
A sequential circuit is different: its output depends not only on the inputs right now but also on a stored *state*, a memory of what came before. Feed a sequential circuit the identical inputs at two different moments and it can give two different answers, because its internal state differs. A counter is the classic case: press the same "tick" and it can show 3 one moment and 4 the next.
The one-question test
Ask: can this circuit give different outputs for the same inputs? If no, the output is fixed by the inputs alone, it is combinational. If yes, it must be holding state, it is sequential. Equivalently, look for a feedback loop or a memory element: combinational circuits are pure feed-forward, sequential circuits contain a loop that stores. That is the whole distinction.
A concrete analogy: a combination lock's digit dial versus a sequence you enter. A calculator's
+ key is combinational, 3 + 4 is always 7. But a turnstile that opens only after the correct sequence of pushes is sequential, the same push means "still locked" or "now unlock" depending on the history. (Ironically, a "combination" lock is really *sequential*: the order matters.)Why this matters practically: it tells you *how to test* the circuit and *whether you need a clock*. A combinational circuit is fully specified by a truth table, and you can check every row in any order. A sequential circuit cannot be captured by a plain truth table, because the output depends on history; you must drive it over time, usually stepped by the clock that paces when the stored state updates.
Do not try to verify a sequential circuit with a static truth table, it will look broken or ambiguous because the same input row maps to different outputs depending on state. Sequential circuits are read as a timing diagram over clock cycles, not as an input-to-output table. Mixing up the two testing methods is the most common early mistake once memory enters the picture.
Check yourself
Sort these into combinational or sequential: (a) an 8-bit adder, (b) a counter, (c) a multiplexer, (d) a register that loads on a clock edge.
Answer
Combinational: (a) the adder and (c) the multiplexer, their outputs depend only on the current inputs. Sequential: (b) the counter and (d) the register, both hold state and can produce different outputs for the same inputs depending on history, so both need a clock to pace their updates.
Frequently asked
What is the difference between combinational and sequential logic?
A combinational circuit's outputs depend only on its current inputs, so the same inputs always give the same output. A sequential circuit also depends on stored state (history), so identical inputs can give different outputs. Sequential circuits contain memory and are paced by a clock.
How do I tell if a circuit is combinational or sequential?
Ask whether it can produce different outputs for the same inputs. If not, it is combinational (pure feed-forward). If so, it holds state and is sequential; look for a feedback loop or a memory element.
Why can't I use a truth table for a sequential circuit?
Because a truth table assumes one output per input combination, but a sequential circuit's output also depends on its stored state, so the same input row can map to different outputs. Sequential circuits are analyzed as timing diagrams over clock cycles instead.
Now that you can spot a circuit that needs memory, build the simplest one. The SR latch is the first sequential circuit, made from cross-coupled gates and feedback.
Every lesson here builds toward one thing: a working CPU, from the transistor up.
Open the free lab →