Although VTAM® masks
most of the differences that exist among the various LU type 0 3270
terminals, situations exist in which an application program can detect
terminal differences. Normally, you should write application programs
to be independent of the terminal differences; otherwise, the program
might have to be modified later when additional LU type 0 3270 terminals
must be supported by the program. This is particularly important in
a multiple-domain environment, in which the application program can
be in one domain and the terminals can be in other domains. The types
of terminal attachment can change in the other domains with little
or no notice to the person maintaining the application program. Application
programs should be coded in the first place to avoid being sensitive
to such changes.
The following items are discussed in more detail earlier
in this
chapter.
For each item, a suggested coding technique is given.
- The queued-response indicator is not supported by the PU type
1 3270. See Responses. Do not use QRI for
LU type 0 3270 terminals.
- The sequence number wraps at 255 for some 3270 LUs and at 65 535
for others. See Sequence numbering and Sequence number dependencies for LU type 0 3270 terminals. Always use the full 2-byte sequence
number for comparisons. Do not become dependent on when the sequence
number wraps, for example, by trying to predict what the sequence
number is when SEND completes.
- Sometimes a negative response for a BB request which was sent
to a BSC 3270 terminal causes entry to the in-bracket state instead
of the between-brackets state. See Figure 1 which
also describes a recovery procedure to use if a subsequent BB request
is rejected. This procedure should be used for all terminals.
- Not all terminals return the same SNA sense codes for particular
error situations. See Exception conditions and sense information. Application
programs should be coded to handle all of the listed codes from any
LU type 0 3270 terminal. The recovery procedure used for each sense
code should be independent of which type of terminal sent the code.
- If an application program disobeys LU type 0 3270 protocols, the
results depend on the terminal involved. See Exception conditions and sense information. Always obey the LU type 0 3270 protocols.
- The copy function is not available for non-SNA channel-attached
3270 terminals. Do
not become dependent on the use of the copy function.
- Do
not use the model number to determine screen size; instead, use information
from the presentation-services field of the session parameter described
in the next item.
- Different 3270 terminals can support different screen sizes and
might support other extended 3270 data stream facilities such as Query.
For further information, see Session parameter.
Application programs can use the presentation-services field of the
session parameter for the terminal to determine such capabilities
for the terminal. Note that this requires the system programmer to
code a logon mode table for the terminal to specify this information.
If a terminal supports Query, that facility can, in turn, be used
to determine details about the capabilities of the terminal. (Refer
to 3174 functional description.)