Configuring Systems for Terminals, Printers, and Other Serial Devices (32022-90057)

Describing Asynchronous Devices
How to Control Printers
Chapter 9
180
How to Control Printers
Controlling printers is 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 speeds on 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 modem is 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 software verifies 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
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.