A table of expected rows

The Test Cases page lets you write expected behaviour as a plain text table — one column per input and output, one row per combination you want to check, using 0, 1, or x for "don't care." Each row runs through its own independent simulation: the listed inputs are driven, the listed outputs are compared, and any row that doesn't match is flagged immediately.

This is exactly a hand-written truth table

A test-case table is nothing more than the truth table you expect your circuit to produce, written down in advance instead of generated automatically afterward — useful the moment a circuit gets big enough that eyeballing the auto-generated truth table stops being a reliable way to catch a mistake.

Why sequential circuits get a warning, not a block

Each row is evaluated independently, with no concept of a clock edge between rows — there's no way to say "toggle the clock, then check the output." If a circuit contains any stateful element (flip-flops, REG4, SHIFT4, any counter), the page shows a warning that row-by-row testing only describes combinational behaviour; it doesn't stop you from testing the combinational portion anyway, since the warning is informational rather than a hard limit.

Try it yourself

Write a small test-case table for a half adder in the circuit editor — four rows covering every combination of A and B — confirm it passes, then deliberately swap two output columns to see how the page reports the mismatch.