HEIDENHAIN TNC 306 Technical Manual - Manual (Page 420)

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TNC 406/TNC 306 1 Introduction 8-7
1.1.3 Checking data
With an asynchronous character frame, transmission errors can be detected by using a parity-check
procedure. A parity bit is sent in addition to the data bits. The evaluation of this bit enables the
receiver to check the parity of received data.
The parity bit can take three different forms; the same form of parity must be set at both interfaces.
– No parity check
Error detection is dispensed with.
– Even parity
The transmitter counts bits with a value of 1. If the number is odd, the parity bit is set to 1,
otherwise it is reset to 0. The sum of the 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 this count
yields an odd number, there is a transmission error and the data word must be repeated, or an error
message will be displayed.
– Odd parity
In this case, the parity bit is so chosen by the transmitter that the total number of all the set bits is
odd. In this case, an error will be detected if the receiver observes an even number of set bits in its
evaluation.
Example:
Letter: "z" 1111010
1 Even parity
0 Odd parity
Parity bit