HEIDENHAIN TNC 306 Technical Manual - Manual (Page 419)

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TNC 406/TNC 306 1 Introduction 3/97
The letter z is represented by the following combination of bits:
1 1 1 1 0 1 0 = 122 dec = 7A hex
MSB LSB
That is, when the letter z is transmitted serially, the appropriate bits are sent one after the other. The
ASCII code is shown in full in the Appendix.
Proper data transmission requires the device concerned to interpret incoming data correctly and, in
particular, to detect the start of a transmission. For this purpose there is a synchronization process
which ensures that the receiver detects the first bit of a character correctly. With an asynchronous
data format, a start bit is sent before each data word and the word is then ended by one or two stop
bits. One feature of this data format is that, starting from a quiescent state, transmission of a data
word can begin at any time.
A quiescent state exists before switch-on and is reverted to after each transmission. Before a data
bit can be transmitted this has to be communicated to the receiver. Otherwise, if the first bit of the
data word has the same value as the quiescent state, the receiver will not notice any difference
from the quiescent state.
A so-called start bit is used for this purpose:
For the duration of a single bit, the transmitter emits a logic value which clearly differs from the
quiescent state and which gives the receiver an opportunity to prepare its polling logic to read in the
data bit. After the start bit has been sent, the data word is transmitted, bit by bit, starting with the
LSB (Least Significant Bit). After the MSB (Most Significant Bit) of the data word, a so-called
parity
bit is inserted (see Section 1.1.3 "Checking data").
The parity bit is followed by one or two stop bits. These final stop bits ensure that the receiver has
enough time to recognize the transmitter again before the start of the next character.
Synchronization is repeated before each character.
Synchronization is repeated before every word and is valid for one character frame.
1st Bit 2nd Bit 3rd Bit 4th Bit 5th Bit 7th Bit 6th Bit Start Parity
bit LSB MSB bit
1 or 2
stop bits
Character frame
Data bits
Quiescent state Quiescent state