Non Return to Zero Inverted (NRZI)

Techopedia explains Non Return to Zero Inverted (NRZI)

Non Return to Zero (NRZ) is a binary code used in telecommunications transmission, where a data bit of 1 is positive voltage, and a data bit of 0 is negative voltage. NRZ code does not have a neutral state, versus Return to Zero (RZ) code, which has a rest state.

In the absence of independent clock signals, certain mechanisms are required when NRZ data is asynchronously coded. NRZI maps binary signals to physical signals during transmission. If a data bit is 1, NRZI transitions at the clock boundary. If a data bit is 0, there is no transition. NRZI may have long series of 0s or 1s, resulting in clock recovery difficulties.

An additional encoding mechanism must be used to ensure clock recovery. Run-length limited (RLL) encoding, such as that used with magnetic disk storage devices, is preferred over Universal Serial Bus (USB) bit stuffing, which often results in variable data transfer rates (DTR).