Non-return to zero encoding is used in slow speed synchronous and asynchronous transmission interfaces. With NRZ, a logic 1 bit is sent as a high value and a logic 0 bit is sent as a low value [really no encoding at all]. The receiver may lose synchronization when using NRZ to encode a synchronous link which may have long runs of consecutive bits with the same value [no changes in voltage]. Other problems with NRZ include; Data sequences containing the same number of 1's and 0's will produce a DC level, and NRZ requires a large bandwidth, from 0Hz [for a sequence containing only 1's or only 0's] to half of the data rate [for a sequence of 10101010].
"A type of 'null' encoding, where a logical 'zero' is represented by a particular line state, and a logical 'one' by another. In other words, there is no encoding, as distinct from RZ encoding." NRZ is used with RS-232 and CANbus.
A code in which 1s are represented by one significant condition and 0s are represented by another, with no neutral or rest condition, such as a zero amplitude in amplitude modulation (AM), zero phase shift in phase-shift keying (PSK), or mid-frequency in frequency-shift keying (FSK).
There are other variations of NRZ encoding; which include:
NRZ-L: [Non-Return-to-Zero-Level]: In NRZ-L encoding, the polarity of the signal changes only when the incoming signal changes from a one to a zero or from a zero to a one. NRZ-L method looks just like the NRZ method, except for the first input one data bit. This is because NRZ does not consider the first data bit to be a polarity change, where NRZ-L does.
NRZ-M [Non-Return-to-Zero-Mark (NRZ-M) Encoding]: The polarity of the signal changes when the incoming signal is a one. An incoming zero would not change the polarity of the signal. A binary encoding scheme in which a signal parameter, such as electric current or voltage, undergoes a change in a significant condition or level every time that a one occurs, but when a zero occurs, it remains the same, i.e., no transition occurs.
NRZ-S [Non-Return-to-Zero-Space (NRZ-S) Encoding]: NRZ-S works just like NRZ-M, except the signal changes when the incoming data signal is a zero, not when the signal is a one. A binary encoding scheme in which a signal parameter, such as electric current or voltage, undergoes a change in a significant condition or level every time that a zero occurs, but when a one occurs, it remains the same, i.e., no transition occurs. The output transitions could also occur only when ones occur and not when zeros occur. If the significant condition transition occurs on each one, the encoding scheme is called non-return to zero mark (NRZ-M).
NRZ-C [Non-Return-to-Zero-Change Encoding]: Same as NRZ-L.
NRZI [Non-Return-to-Zero-Inverted Encoding]: A '0' is encoded as no change in the level. However a '1' is encoded depending on the current state of the line. If the current state is '0' [low] the '1' will be encoded as a high, if the current state is '1' [high] the '1' will be encoded as a low. Used with FDDI and USB for example.
NRZ1 [Non-Return-to-Zero Change-on-Ones]: A code
in which 1s are represented by a change in a
significant condition and 0s are represented by no
NRZ-1 [Non-Return-to-Zero One]: Same as NRZ-M.