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3 Interface Modules
06.91
3.3.3 XON/XOFF procedure
3.3.3 XON/XOFF procedure
The XON/XOFF line procedure is a character-oriented transmission method. The receiving
system controls transmission of data of the transmitting system by means of XON/XOFF. Data
is generally transmitted in half-duplex mode and in blocks. The user need not take into
consideration block lengths or data overflow.
XON/XOFF is generally represented by the control characters DC1/DC3 (11H/13H); however,
both control characters are user-configurable. The DTR, DSR, RTS and CTS control and
signal lines need not be connected for the XON/XOFF procedure. The DSR signal line is an
exception. The evaluation of the DSR line (Data Set Ready) depends on the configuration. If
the parameter DSR evaluation has been initialized with "with", the DSR status is interrogated
when transmitting the first character.
The XON/XOFF procedure distinguishes between two (configurable) variants.
3.3.3.1 Communication between "intelligent" and "non-intelligent"
(unequal) nodes
The receiving station sends the XON character after initiation of the "Data Start" command. As
a precondition, the receiving station must be configured as a slave.
The transmitting station must wait for the XON character to be transmitted before starting
transmission. For this scope of functions, an additional configuring parameter is available -
namely, "Waiting for 1st XON". "Waiting for 1st XON" is independent of the priority of the
station.
After the XON character has been sent by the receiving station, the station on the other end
starts transmission. If transmission is terminated by a valid end criterion, another transmission
can be initiated only by the "Data Start" command.
The receiving station sends XOFF if it is no longer ready to receive. With regard to the printer,
this would be the case if the print buffer is full or the OFF line key is pressed or if the printer
enters the faulted state.
The CP 315 sends an XOFF character if the receiving scratch (input buffer for the respective
procedure) is disabled, e.g. in the case of equipment bottlenecks, or if the receiving scratch is
full or previous jobs cannot be passed on due to missing acknowledgements.
The station sends an XOFF character even if the interface is not yet assigned.
3–20
© Siemens AG 1989 All Rights Reserved
6FC5 197-0AB20-0BP0
SINUMERIK 840/880 (PJ)
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