Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)

Chapter 2 45
Controlling Asynchronous Devices Programmatically
Device Operation Modes
At the completion of the program, control of the terminal is returned to
the active session. For this reason it is critical that any device
characteristics altered by the program be reset to normal or default
settings before the session regains control. If a program fails to reset
altered characteristics, the session may not be able to communicate
successfully with the MPE/iX system.
NOTE For similar reasons, it is strongly recommended that you disable system
break through use of the FCONTROL system intrinsic in any program
which significantly alters the operating characteristics of a device. If
system break is not disabled and the
[BREAK] key is pressed during
execution of such a program (one that uses block mode, for example),
the device may not be able to resume normal operations. Both of these
recommendations are discussed in greater detail in later chapters of
this manual.
Programmatic Devices
Programmatic devices are those which run under the control of a
program running on the computer. The program accesses the device
through the File System by treating the device as a file. The File
System passes the information to the ASC software, which
communicates with the device itself.
A programmatic device is considered to be program captive, because
access to the device is through the program only. For instance, no
MPE/iX commands can be executed from a terminal under the control
of a program unless the controlling program initiates the command.
Programmatic devices can be used for input, output, or both, depending
on the device and how it was opened by the controlling program.
There are many situations in which devices that are operating in one of
the other device operation modes are also operating programmatically.
As explained in the discussion above, session-accepting devices become
programmatic devices when their users execute a :RUN command.
The system console can be a programmatic device in the same way.
Slaved and spooled devices can be considered to be programmatic in the
sense that they must be controlled through an application.