modulation nrz
Non-Return-to-Zero 'does what it says on the tin': the signal does not return to zero unless it has to. If a string of 1s is present, the signal stays at the level corresponding to a 1 for the whole diuration of the string of 1s. This means it is (by itself) *not* good for e.g. magnetic data recording because it is very difficult to recover the clock (which can only be done from transitions) unless the data itself is coded to force reasonably frequent 0-1 or 1-0 transitions. NRZ would commonly be used where there is another source of clock, or where the data is coded to make clock recovery easy. RS232 uses NRZ, and enables clock recovery by enforcing a 1-0 transition from idle at 1 to the start bit at 0, but then only has transitions when the data changes, until ending at 1 for the stop bit(s) within 7/8/9 bit periods (depending on 7/8 databits, and optional parity bit). The only guaranteed transition is the start bit, and UARTS typically use 16x over-sampling to get acfcurate sampling of the data bits based on the timing of the strat-bit transition, which works fine at RS232 data rates upto 115kbits/s.
By contrast Manchester encoding guarantees at least one transition on every bit (so clock recovery is very easy), and MFM encoding every 1/1.5/2 bits (so clock recovery isn;t too hard).