STBREAK - Set Break

Use the STBREAK macro instruction to indicate whether the transmit interrupt feature on an IBM® 1050, 2741, 3270, 3767, or 3770 terminal will be used or suppressed. The transmit interrupt feature lets terminal output processing interrupt terminal input processing.

The transmit interrupt feature is a special feature on 1050 and 2741 terminals; it is a standard feature on the 3767, 3770, and 3270 display terminals. Specifying STBREAK YES for a 1050 without the transmit interrupt feature could result in loss of output or a permanent error at the terminal.

When the transmit interrupt feature is being used by the system, the terminal user can enter the next line while the previous one is being processed. All 33/35 Teletypes and IBM 3270, 3767, and 3770 terminals are handled this way. 1050s and 2741s that have been defined in the message control program as having the transmit interrupt feature will be handled this way unless STBREAK NO is specified.

Note: For 2741s, 3767s, 3770s, TWX, and WTTY devices supported by VTAM®, the keyboard will remain unlocked when STBREAK NO is specified.

When the feature is in use, terminal handling of input and output is as follows: if no output is available for the terminal, and if there are sufficient TSO/E terminal buffers available, the keyboard will be unlocked to allow the user to enter input. If the user's program generates output (TPUT) before he has started to enter data, the read operation is halted and the break (transmit interrupt) feature can be used to lock the keyboard and condition the communications line to transmit output. If the user has already started to type when the TPUT is issued, the output will not be sent until he has finished that line of input. If, however, the TPUT had specified the BREAKIN option, the output message would interrupt any input in progress. If the application does not issue a TCLEARQ macro to erase the contents the input buffer queue then,

The interrupted input from a 1050 or a 2741 terminal will be printed out again after the output is sent, to let the user continue to type from the point where he had been interrupted.
The interrupted input from a 3767, 3270, or a 3770 terminal is received by the application program but is not printed at the terminal.

When the transmit interrupt feature is not being used by the system, a 1050 or 2741 terminal keyboard is unlocked only after the user's program has issued a TGET request for input. (A 3270, 3767, or 3770 terminal keyboard's normal state is unlocked.) In this mode of operation, the terminal user cannot type ahead of his program. A TPUT with the BREAKIN option cannot interrupt input. The output will not be sent until the terminal user has completed entering his current input line. All display stations are handled in this way. All 1050s and 2741s that have been defined in the message control program as not having the transmit interrupt feature are handled this way.

The STBREAK macro instruction can be used only in a time sharing environment. It is ignored if TSO/E is not active when the macro instruction is issued.

Figure 1 shows the format of the STBREAK macro instruction.

Figure 1. The STBREAK macro instruction

 [symbol]    STBREAK         [YES]
                             [NO ]

YES | NO

YES: indicates that the transmit interrupt feature will be used. YES is the default.
NO: indicates that the transmit interrupt feature not be used.

When control is returned to the user, register 15 will contain one of the following return codes:

Table 1. Return codes from STBREAK
Return code dec(Hex)	Meaning
0(0)	Successful.
4(4)	An incorrect parameter was specified.
8(8)	The terminal type is not valid. This macro instruction should be issued only for the IBM 1050, 2741, 3270, 3767, or 3770 terminal.