Summary of differences among LU type 0 3270 terminals

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.)