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

Chapter 9 199
Describing Asynchronous Devices
How to Control Printers
Status Request
Printers using printer type files 21, 22, or 26 have a data protection
mechanism that prevents the DTC from sending data to a printer when
it is unable to print it. This mechanism is called status request or
status checking. Printers respond to the request and return
information on their status. A status request is sent after each file has
printed.
Some printers are unable to return status information to the ASC
software or to the user program controlling it. These printers should
use printer type file 18, which does not send status checks to printers.
Since the ASC software has no way of knowing when something is
wrong, data can be lost.
Vertical Format Control
Many printers use Vertical Format Control (VFC) to direct the
placement of output. VFC instructs a printer to skip to predetermined
lines or to a specific point on a page with typically one or two
commands, instead of counting and outputting a number of blank lines
and spaces. This capability is often referred to as “Skip to Channel x,”
where x is one of the predetermined points.
Printer Initialization
When a printer is shared, each user can manipulate the printer in a
different way. One user can change the printer’s characteristics (such as
margins, tabs, or print density) and affect another who does not want
those characteristics.
To prevent one user from affecting other users, the printer must be
initialized to a known state before printing the next job. When a printer
configured as printer type file 21, 22, or 26 is opened, the ASC software
writes an initialization character string to the printer.
Printers using printer type file 18 cannot use the initialization string.
This means that if a user changes the printer’s characteristics, the
characteristics are not reset by the ASC software. The user’s program
must reset the printer when it is finished with it.