Non return to zero, inverted (NRZI) is a method of mapping a binary signal to a physical signal for transmission over some transmission media. The two level NRZI signal has a transition at a clock boundary if the bit being transmitted is a logical 1, and does not have a transition if the bit being transmitted is a logical 0.

“One” is represented by a transition of the physical level, while “zero” has no transition. Also, NRZI might take the opposite convention, as in Universal Serial Bus (USB) signalling, when in Mode 1, in which a transition occurs when signaling zero, and a steady level when signaling a one. The transition occurs on the leading edge of the clock for the given bit. This distinguishes NRZI from NRZ-Mark.

However, even NRZI can have long series of zeros (or ones if transitioning on “zero”), and thus clock recovery can be difficult unless some form of run length limited (RLL) coding is used in addition to NRZI. Magnetic disk and tape storage devices generally use fixed-rate RLL codes, while USB uses bit stuffing, which inserts an additional 0 bit after 6 consecutive 1 bits, thus forcing a transition. While bit stuffing is efficient, it results in a variable data rate because it takes slightly longer to send a long string of 1 bits than it does to send a long string of 0 bits.