Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90051)

180 Chapter9
Describing Asynchronous Devices
How Operation Modes Set Functionality
• Place the terminal under partial control of a program running on the
computer.
• Communicate with other terminal users and with the system
operator.
• Transfer data and control information between the computer and a
slaved device connected to the terminal.
• Change the line speed of the terminal.
• Terminate the session.
Also, session-accepting devices that are not connected through a PAD or
Telnet/iX and are not configured for switching can temporarily obtain
partial system console functionality by using the CONSOLE command. A
terminal that is the logical console can receive console messages,
execute commands that do not require
[CTRL]-A, and return console
control to the system console (through the CONSOLE command).
Programmatic Devices
Programmatic devices run under the control of a program on the
computer. The program accesses the device through the file system by
treating the device as a file (device file). The file system passes
information from the controlling program to the software which
communicates directly with the device.
A programmatic device is captive of the program because access to the
device is through the program only. For instance, MPE/iX commands
cannot be executed from a terminal under the control of a program
unless the controlling program initiates the command. A programmatic
device can be used for input, output, or both, depending on the device
and how the controlling program opened it.
Spooled Devices
MPE/iX is equipped with a spooling facility to assist in the
management of nonshareable devices. When a spooler process controls
a nonshareable device, the device appears to be shared among several
users, when in fact, it can process only one file at a time. This is done by
temporarily storing data on disk instead of sending it directly to the
device. The disk essentially becomes a staging area, while the spooler
manages the selection of output spool files destined for the spooled
device.
A user program writing data to a spooled output device actually writes
records to a disk file instead of to the device itself. When the output
device is available, the spooler process selects a file and begins to send
the data in the file to the device.