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Flow of Data from the SNA Host
Data received from the SNA host over the SDLC link is placed directly into the
system buffers. The data is checked for errors and the SNA headers and trailers are
removed. As a chain of elements are received, the data is transferred to the device
buffer. For segmenting, when all of the segments have been received, the data
stream is processed and the data is transferred to the device buffer. The data is
translated from EBCDIC to ASCII when it is transferred to the device buffer. In the
device buffer, a screen image is formed from the data. For LU type 2 displays and
LU type 3 printer sessions, the data is received in 3270 data stream format. After
the entire data stream (a complete SNA chain) has been received, the data is written
to the terminal.
If a sequence is needed to position the cursor on a display or move the print head on
a printer, the sequences are built in the write queue. For displays, these sequences
are created from the terminal tables that are in storage. (Refer to IBM 3708
Network Conversion Unit Planning and Installation for all of the default terminal
tables.)
For LU type 2 sessions, host changes to the terminal screen image are tracked. Only
changed fields or lines are written to the terminal.
For more information about protocol conversion data streams for LU type 1, type 2,
and type 3, see "3270 Data Stream (LU type 2)," "3270 Data Stream (LU type 3)"
on page 4-15, and "3270 Data Stream (LU type 1)" on page 4-19.
Flow of Data from the Terminal
As data is received from the terminal, it is placed in the receive queue (the size of
which is configurable in the 3708). From the receive queue, the data is processed,
and as a result, may be copied into the device buffer and echoed back to the
terminal (the processing, called key tracking, is described in "Key tracking" on
page 4-77). When the end user presses an AID key (for example, Enter), the data is
moved from the device buffer, converted to a 3270 data stream and to the system
buffers. In the system buffer, the SNA trailers and headers are added to the data
stream and the data is sent to the host. For LU type 2 sessions, terminal changes to
the terminal screen image are tracked. This allows the 3708 to send just the changed
fields, if requested.
3270 Data Stream (LU type 2)
When an LU type 2 BIND is received from the host, the 3708 is operating in
protocol conversion mode.
Note: The 3708 can operate in all three modes (protocol conversion, protocol
enveloping, and ASCII pass-through) simultaneously. The BIND from the host and
the 3708 configuration determine in which mode a single port operates.
When operating in protocol conversion mode, the 3708 accepts commands, orders,
attributes, and data from the host. The 3708 processes the data and sends it to the
ASCII display in the form of data and terminal control codes. The following section
describes all of the 3270 data stream commands, orders, and attributes that the 3708
accepts. For more detailed information about the 3270 data stream, refer to the
IBM 3270 Information Display System Data Stream Programmer's Reference,
GA23-0059.
Chapter 4. Protocol Conversion Mode
4-3
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