Serial Formats

When sending data over a radio link, there are many stages that are capacitively coupled. Therefore, it is important to use a data encoding method that results in a 50% average value. The following extract is from an old Zilog data book.

Data Encoding
NRZ: a 1 is represented by a High level and a 0 is represented by a Low level.
NRZI: a 1 is represented by no change in level and a 0 is represented by a change in level.
FM1: (bi-phase mark), a transition occurs at the beginning of every bit cell. A 1 is represented by an additional transition at the center of the bit cell and a 0 is represented by no additional transition at the center of the bit cell.
FM0: (bi-phase space), a transition occurs at the beginning of every bit cell. A 0 is represented by an additional transition at the center of the bit cell, and a 1 is represented by no additional transition at the center of the bit cell.
Manchester: (bi-phase level), there is always a transition at the center of the bit cell. If the transition is 0 to 1, the bit is a 0. If the transition is 1 to 0, the bit is a 1.

From Zilog Z80C30/Z85C30 data sheet (corrected).