Synchronization; Transmission Reliability: Parity Bit - HEIDENHAIN TNC 426 PB/M Technical Manual

Synchronization

Transmission
reliability: Parity bit
Example
8 – 6
A synchronization process ensures that the receiver correctly detects the first
character of a transmission.
With an asynchronous character frame, the transmission of a data word can
begin at any time, starting from an idle state.
A start bit is transmitted before each data word. If the first bit of the data word
had the same value as the idle state, the receiver would not notice any
difference from the idle state.
After the start bit has been sent, the data word is transmitted, bit by bit,
starting with the LSB (Least Significant Bit). The MSB (Most Significant Bit) of
the data word is followed by the so-called parity bit in order to detect
transmission errors.
The character frame is concluded by one or two stop bits. The stop bits enable
the receiver to recognize the transmitter again before the start of the next
character.
Synchronization is repeated before each character and applies for one
character frame.
With an asynchronous character frame, transmission errors can be detected
by using the parity bit.
The parity bit can take three different forms.
n
No parity check: Error detection is dispensed with.
n
Even parity:
The transmitter counts bits with a value of one. If the number is odd, the
parity bit is set to one, otherwise it is cleared to zero. The sum of set data
bits and the parity bit is therefore always even. Upon receiving a word, the
receiver counts all of the set bits, including the parity bit. If the count is odd,
there is a transmission error and the data word must be repeated, or an error
message will be displayed.
n
Odd parity:
The parity bit is so chosen by the transmitter that the total number of all the
set bits is odd. An error will thus be detected if the receiver observes an
even number of set bits in its evaluation.
The letter "z" corresponds to the bit sequence: 1 1 1 1 0 1 0
Parity bit
n
with even parity = 1
n
with odd parity = 0
HEIDENHAIN Technical Manual TNC 426, TNC 430