Table of Contents
Advertisement
Flow of Data from the SNA Host
As data is received from the host, the 3708 collects it in system buffers. Each system
buffer holds a
256~byte
RU. Data can be received as chain elements or segments. If
data is segmented, all segments must be assembled before data is transmitted.
Once a chain element has been received and processed (or once all the segments have
been assembled), the data is moved from the system buffers into the 3708 device
buffer. If the 3708 is configured to translate data, it is done at this time. As the
data is processed, printer delay characters (CR delay or FF delay) are inserted into
the device buffer when appropriate. The data is processed until all of the chain
element has been processed, or the device buffer is filled. (The device buffer is 2000
bytes).
Once the data has been translated and the data stream built, it is transmitted to the
downstream device. For HDX control units, the data is not transmitted until any
receive operation currently in progress is completed (completion is indicated by
receipt of a line turnaround character or a text timeout).
After an entire host message has been transmitted to the downstream device
(indicated by an only-in-chain or end-of-chain), a positive response is sent to the
host for that chain if a defined response was requested in the SNA header. After the
host message has been transmitted and if no more host data is waiting to be sent, the
3708 checks to see if a read prompt is defined. If a read prompt is defined, the read
prompt message is translated (if specified) and sent to the downstream device. The
read prompt completes this host transmission and indicates that data entry is
expected.
Flow of Data from the Terminal
Once the line is enabled, the 3708 is always in receive state. Data from the
downstream device is received into the receive queue. The receipt of data causes the
text timer and inactivity timer to be restarted.
The 3708 processes the data from the receive queue unless it encounters one of the
following conditions:
• The 3708 is in buffer slowdown.
• The host has indicated a busy condition (RNR received from host).
• Echoplex is specified and host data is currently being transmitted.
More than I character may be in the receive queue before it is processed. As data
from the downstream device is processed off the receive queue, it is accumulated into
system buffers. The following conditions cause the 3708 to forward the accumulated
data to the host:
• A system buffer is filled (256 byte RU).
• A line turnaround character is received.
• A text timeout occurs.
Data bytes are removed from the receive queue, I byte at a time for processing. If
echoplex is configured, each received byte is copied to the write queue (which is 40
bytes long) for transmission to the downstream device.
5-12
IBM 3708 Description
Advertisement
Table of Contents
