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

20 Chapter1

Introduction

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.”