HP 3000 Manuals HP 3000 Manuals
How to Control Printers
Controlling printersis similar to controlling terminals, except for two
differences:
* Printers have no need for control mechanisms (such as echo,read
trigger,special characters,or modes of input) related to user
input. Note: printers which support status checking require a
read trigger.
* Printers need additional control mechanisms to define how output
will appear on the printed page, as well as special ways to
prevent data loss.
Each of the methods for controlling printers is explained in the pages
that follow. Refer to the Asynchronous Serial Communications
Programmer's Reference Manual for more information on programmatic device
control.
Transmission Speed
Asynchronous data transmission speedson MPE/iX computers range from 300
bps to 19200 bps. The following speeds are supported:
300, 1200, 2400, 4800, 9600, and 19200 bps. 38400 bps is also supported
for direct connect devices on DTC 16iX/16MX and DTC 72MX.
If host-based, transmission speed is configured through NMMGR.The speed
chosen
must be supported by the printer. A printer can be changed to any
supported speed if it is directly connected to the DTC. The speed of a
printer connected to a modemis set by the speed of the modem and the
telephone line being used.
Parity
Parity verifies that each character of data is transmitted between the
system and the printer without error. It detects data transmission
errors, but does not correct them. Printers using printer type file 21
use parity. Printer type file 21 generates odd parity.When a printer
using printer type file 21 encounters a parity error,it remembers the
error until the ASC softwareverifies the printer's status. When the
error is returned to the ASC software, a message is printed on the system
console.
Printers using printer type files 18 or 22 cannot use parity.
Additionally, the FCONTROL and the FDEVICECONTROL intrinsic cannot be
used to change the parity settings for printers. For more information on
parity,refer to "How to Control Terminals" earlier in this chapter.
Status Request
Printersusing printer type files 21, 22, or 26 have a data protection
mechanism that prevents the DTCfrom sending data to a printer when it is
unable to print it. This mechanism is called status requestor status
checking.Printers respond to the request and return information on their
status. A status requestis sent after each file has printed.
Some printers are unable to return status information to the ASC
softwareor to the user programcontrolling it. These printersshould 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'scharacteristics (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
softwarewrites an initialization character stringto 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.