Non return to zero
Non-Return-to-Zero and Non-Return-to-Zero-Inverted or alternating letters , abbreviated NRZ and as NRZI , are the simplest line codes for binary signals. In contrast to the RZ code , the two binary symbols consist of constant line states (mostly voltages ). The disadvantage is that the recipient becomes uncertain about the length of the sequence when transmitting a longer sequence of the same symbols. A separate clock signal is required as with the I 2 C bus, a frame formation as with EIA-232 , the use of scramblers as with SDI or an additional run length-limiting line coding such as bit stuffing .
The term non-return-to-zero does not refer to a possibly impermissible voltage value of 0 V, but rather that there is no third voltage value that is applied for part of each symbol duration, as is the case with RZ. Another interpretation says that the voltage in the middle of the bit can never drop back to the value 0.
NRZ
The NRZ code directly assigns a line status to each bit value. It can easily be used if there are no long constant sequences in the user data, as is the case with ASCII- coded texts. The limit for 'long' can be quite short, for example for a tape drive with wow and flutter .
The NRZ coding is generally not free of equal components and is therefore problematic in particular with magnetic data recording . A simple galvanic isolation in the signal transmission path by means of pulse transformers is therefore not possible.
UARTs e.g. B. use the NRZ coding.
NRZI
The NRZI coding ( Non Return to Zero Inverted ) assigns the line status to one of the two bit values and a status change ( inversion ) to the other bit value . This immediately results in freedom from polarity: Reverse polarity of the transmission line does not change the bit sequence.
NRZI exists in two variants, depending on whether ones ( Mark ) or zeros ( Space ) cause a status change. If d k is the data sequence at the input and p k is the level sequence at the output, the formation rule for NRZ-M is:
and for NRZ-S:
The operator denotes the modulo-2 addition, which can be implemented with an exclusive-or gate , k −1 the previous value (e.g. from a latch ) and the overline - a negation (for NRZ-S).
The NRZI coding can be used without further ado if it is known that the useful data do not have long sequences of zeros (NRZ-M) or ones (NRZ-S). At the beginning and at the end of the user data, bits that do not change the line status can be recorded with a synchronization frame.
NRZI is used for USB , for Ethernet over fiber optics (100BASE-FX) and for FDDI . NRZI is also used when recording data on storage media such as CD-ROM or hard drives .
NRZ-M
NRZ-M causes (rarely NRZI-M) a bit change at one, see examples. A zero does not cause a bit change.
| Example 1: | |
| Data bits (logical): | 1 1 1 1 1 1 1 1 |
| physical line in initial state "1": | 0 1 0 1 0 1 0 1 |
| physical line with initial state "0": | 1 0 1 0 1 0 1 0 |
| Example 2: | |
| Data bits (logical): | 0 0 0 0 0 0 0 0 |
| physical line in initial state "1": | 1 1 1 1 1 1 1 1 |
| physical line with initial state "0": | 0 0 0 0 0 0 0 0 |
| Example 3: | |
| Data bits (logical): | 1 1 1 1 1 0 1 0 1 0 1 1 0 0 0 1 |
| physical line in initial state "1": | 0 1 0 1 0 0 1 1 0 0 1 0 0 0 0 1 |
| physical line with initial state "0": | 1 0 1 0 1 1 0 0 1 1 0 1 1 1 1 0 |
NRZ-S
NRZ-S (rarely NRZI-S) causes a bit change at zero, see examples. A one does not cause a bit change (USB).
| Example 1: | |
| Data bits (logical): | 1 1 1 1 1 1 1 1 |
| physical line in initial state "1": | 1 1 1 1 1 1 1 1 |
| physical line with initial state "0": | 0 0 0 0 0 0 0 0 |
| Example 2: | |
| Data bits (logical): | 0 0 0 0 0 0 0 0 |
| physical line in initial state "1": | 0 1 0 1 0 1 0 1 |
| physical line with initial state "0": | 1 0 1 0 1 0 1 0 |
| Example 3: | |
| Data bits (logical): | 1 1 1 1 1 0 1 0 1 0 1 1 0 0 0 1 |
| physical line in initial state "1": | 1 1 1 1 1 0 0 1 1 0 0 0 1 0 1 1 |
| physical line with initial state "0": | 0 0 0 0 0 1 1 0 0 1 1 1 0 1 0 0 |
See also
- Modified Frequency Modulation (MFM)
- Group Coded Recording (GCR)
literature
- John G. Proakis, Masoud Salehi: Communication System Engineering . 2nd Edition. Prentice Hall, 2002, ISBN 0-13-095007-6 .
