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Processing the 3270 Data Stream
When operating in protocol conversion mode, the 3708 analyzes the LU type 2 data
stream (see "3270 Data Stream (LU type 2)" on page 4-3), building a screen image
in the 3708 device buffer for the port (see "Display Image" on page 4-29 for a brief
description of the device buffer).
The order of processing is:
1.
If the command is a Write and the WCC indicates that MDT bits are to be
reset, the device buffer is searched for attribute bytes, and the MDT bit (bit 7) is
set to O.
2. If the command is an Erase/Write or an Erase/Write Alternate, the device buffer
is cleared to nulls.
3. If the terminal is enabled for large screen support, the 3708 may transmit a
sequence to the terminal to set the screen size.
• If the terminal is not in the default screen size and an E/W is received, the
3708 transmits a sequence to the terminal to set its screen size to the default
size specified in the BIND.
• If an E/W Alternate is received and the terminal is not in alternate screen
size, the 3708 transmits a sequence to set the terminal in the alternate screen
size specified by the BIND.
4. The host data is analyzed for orders and data, and from these, the device buffer
is updated to represent the display image. For each line of the display image,
two column-number values are maintained. These are used to contain the
rightmost and leftmost column positions that have been modified by the current
Write command from the host. These values are used later when transmitting
the data to the terminal.
5. An entire Write command is received and analyzed by the 3708 before any
transmission to the terminal occurs. If the data has been split into chain
elements, transmission to the terminal begins only after the element specifying
End Chain has been received.
6. If the Type-ahead function has been activated, a positive SNA response is sent,
if appropriate. The queue of characters received from the terminal is then
checked. If data has been received, it is processed and (possibly) added to the
device buffer data. When an AID type key sequence is detected in the received
data, the appropriate indication is transmitted
to
the host program, and the
receive queue processing terminates.
Chapter 4. Protocol Conversion Mode
4-13
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