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3 Interface Modules
3.3.2 LSV2 procedure
The following control characters are used:
ENQ
Enquiry
The ENQ data transmission control character is transmitted to a station as "request to
respond". ENQ requests the receiver to send an acknowledgement character.
DLE
Data Link Escape
With the DLE data transmission control character it is possible to give another meaning
to a limited number of directly following characters. In conjunction with characters 0
and 1 the character combinations DLEO and DLE1 are used as positive
acknowledgement. Due to the DLE control character, the next character is interpreted
as control character if it has been coded as data transmission control character. If DLE
occurs as character in the text, it has to be doubled and is interpreted as text
character.
In the receiving station this is undone. If a single DLE occurs in the text with any
character, the block is rejected as incorrect and acknowledged with NAK.
DLE/
Block abort
ENQ
If the transmitting station sends DLE/ENQ anywhere in the text, this means that the
block is aborted. The receiving station must respond with NAK.
DLE/
Start of Text
STX
The DLE/STX transmission control character directly precedes the text. During block
securing, the adding procedure to obtain the block check character on STX begins
with resetting the block check unit to ZERO.
DLE/
End of Text
ETX
DLE/ETX signifies the end of a text and indicates that the next character is a block
check character (BCC). ETX is always included in the adding process for block
checking.
EOT
End of Transmission
This transmission control character is used in order to identify the end of transmission
of texts. The text transmitting station sends EOT in order to either indicate
transmission end or abort. The transmitter abandons control of transmission. Both
stations (text transmitting and receiving stations) return to the basic phase.
NAK
Negative Acknowledgement
The text receiving station sends NAK as negative acknowledgement to the text
transmitting station. This is an acknowledgement to a request to receive in order to
indicate that the station is not ready to receive. NAK is sent during the text transfer
phase as acknowledgement to the reception of a transmitted text if the transmission
has not been received error-free and the text receiving station is ready to receive this
block once again.
3–18
© Siemens AG 1989 All Rights Reserved
08.89
6FC5 197-0AB20-0BP0
SINUMERIK 840/880 (PJ)
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