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

38 Chapter1
Introduction
Data Communications Concepts Reviewed
The DTC is protected from overruns through an additional flow control
mechanism referred to as the read trigger. The read trigger is also the
ASCII DC1 character, and is generated by the ASC software when the
system is ready to accept data.
There is also a flow control mechanism used between a system and its
DTCs called the Transport Flow Control Protocol. Its fundamental
purpose is to protect DTCs from being overrun by data from the system.
It also handles error checking of the packets passed to and from the
system through the LAN. This flow control mechanism does not directly
affect the way devices act and react.
NOTE The characters XON (ASCII DC1) and XOFF (ASCII DC3) are defined
as special characters and are reserved for the purpose of protocol. These
characters may not be used as data except in binary mode, if binary
mode is supported.
Local Device Settings
Each device has certain specific characteristics which define the way in
which it physically interacts with the system. For HP terminals these
characteristics are generally set by changing configuration parameters
from the keyboard via terminal configuration menus. Terminal settings
include such things as baud rate (transmission speed), parity settings,
and terminal block mode functions. Terminal settings can usually be
altered programmatically, through the use of appropriate escape
sequences, as defined for each type of terminal.
You may hear the term strap setting used synonymously with device
configuration setting. This is left over from a time when changing a
device’s characteristics meant physically opening its cover and moving
a strap from one set of contacts to another. A number of settings are
still referred to by their original alphabetic strap designations. For
example, how you set the D, G and H straps (Line/Page, InhHndShk
and Inh DC2 settings respectively) determines how your terminal will
respond to block mode processing.
Certain printer characteristics are also set locally at the device and
may also be altered by transmission of appropriate escape sequences.
Though printers have no need for all of the control mechanisms that are
related to user input, such as echo, or block mode, they do have their
own special set of control needs to determine how output appears on the
printed page, and to define ways to prevent a loss of data.
An important distinction must be made between the physical setting of
the device (what the device does and expects) and the programmatic
setting of the ASC software (what the system does and expects). The
two settings must agree with each other in order for data transmission
to take place. If you programmatically alter the system setting through
an intrinsic call, you must also make sure the device’s physical setting