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

Introduction
Devices that Communicate Asynchronously
Chapter 1
20
Devices that Communicate Asynchronously
Asynchronous Serial Communications (ASC) is the term used to describe the
specific manner in which communications occur between a HP 3000 Series 900 computer
and some of its associated devices. Asynchronous refers to the protocol used to inform the
receiving end of a communications link that data is being sent and when the data
transmission has completed. Each character is preceded by a special bit, called a start
bit, which signals its arrival. The character is then followed by at least one stop bit, (a
second special bit) that signals transmission of that character is complete.
The types of devices that communicate asynchronously include the following:
•Terminals
Personal computers (PCs) in terminal emulation mode.
Serial printers
•Plotters
Modems and other devices that use modem signals.
HP PowerTrust UPSs (Uninterruptible Power Supplies) (herein referred to as
UPSs).
This asynchronous communication scheme supports the following device
functionality:
Device control capabilities provided through the FCONTROL and FDEVICECONTROL file
system intrinsics.
Typeahead facility, which allows terminal users to enter data before a read is posted.
Block mode applications.
Customized terminal and printer type files created with the workstation
configurator utility.
Datacommunications and Terminal Controller (DTC) switching capabilities.
Powerfail session recovery for sessions in character mode (that is, switched,
non-switched, nailed, and non-nailed sessions).
NOTE For more information about these device functionalities, refer to Chapter 9, Describing
Asynchronous Devices.”