HP 3000 Manuals

COMMUNICATOR 3000/XL

Chapter 6  DATA COMMUNICATIONS 

Introduction to MPE XL Asynchronous Serial Communications 

by Bob DePeyster and Mike Genevro--Information Networks Division 

A wide range of useful peripheral devices are available for use with your
HP 3000 Series 900 computer.  Each type of peripheral device communicates
and interacts with the computer in a very specific way.  The way that
this interaction occurs depends on such diverse factors as the speed of
the device, the design of the computer's operating system, and the
purpose for which the peripheral device is used.  Important factors to
keep in mind are that the computer and the peripheral device must agree
on the method of communications being used, and they must know exactly
what to expect from each other.

This article deals with one set of peripheral devices and the method
through which they communicate with the MPE XL operating system.  The
devices discussed here are the terminals and printers that provide users
with their most direct means of communication with the computer.

ASYNCHRONOUS SERIAL COMMUNICATIONS 

Asynchronous Serial Communications (ASC) is the technology that includes
all the hardware and software necessary for successful communications to
take place between the computer and its associated terminals and
printers.  Asynchronous Serial Communications also describes the method
used for such communications.

Asynchronous refers to the scheme used to let the receiving end of a
communication link know when data is being sent and when transmission of
the data has completed.  Each character is preceded by a special bit,
called a start bit, which signals its arrival.  The character is then
followed by a second special bit, a stop bit, which signals that
transmission of that character is complete.  Because this scheme allows
data to be sent intermittently and does not require elaborate timing
mechanisms, the asynchronous method is particularly useful for reading
characters entered through a keyboard, where the time interval between
the entry of two characters can vary greatly.

Serial transmission refers to the way each character is transferred.
Characters transmitted serially are sent one bit at a time and received
one bit at a time in the order of transmission.  The figure labeled
"Asynchronous Serial Transmission" depicts transmission of data
characters in an asynchronous serial mode.


          Asynchronous Serial Transmission

For more information about the methods used to transmit data between two
devices, refer to Connecting to Your Computer (P/N 5957-4625), a
workstation-to-computer communications primer.

Devices 

The devices that communicate with MPE XL asynchronously include a number
of supported terminals and printers.  Modems may also be a part of the
communications link.  The characteristics of each device are determined
by the MPE XL operating system as well as by the device itself.

Terminals 

A terminal is a hardware device that enables an end-user to access the
computer directly to input data and receive output.  All terminals
supported for use on MPE XL systems include typewriter-like keyboards
and CRT screens.  Some personal computers capable of running in
terminal-emulation mode are also supported.

Terminal Types are assigned to each terminal connected to the system and
serve to help define the terminal's features.  For a complete
discussion of Terminal Types, see Chapter 4 of the Asynchronous Serial
Communications System Administrator's Reference Manual (P/N 32022-90001).

Printers 

Like terminals, printers can be connected to the system through ASC.
Unlike terminals, printers are used only to receive information and to
transfer that information to paper in a permanent, readable format.
Printer Types are used to help define the characteristics of each printer
to MPE XL.

Modems 

In some cases asynchronous communications may take place over
telecommunication lines.  Because the signal used by computers (digital
signal) is different from the signal transmitted over telephone lines
(analog signal), a device called a modem is required on each end of such
a link.  The word "modem" comes from "modulator/demodulator" , which
describes the function of the device.

Details on all supported modems and their recommended settings can be
found in Chapter 5 of the Asynchronous Serial Communications System
Administrator's Reference Manual (P/N 32022-90001).

System Console 

The System Console is a terminal used by the system operator to execute
specific tasks for the purpose of managing sessions, jobs and system
resources.  It is connected to the system through its own access port,
and its functions are controlled through the Console Management software.
Although the System Console uses the same ASC communication method as an
MPE XL terminal, the System Console is not a part of the DTS. See Chapter
2 of the Asynchronous Serial Communications System Administrator's
Reference Manual (P/N 32022-90001) for more information about the System
Console.

DISTRIBUTED TERMINAL SUBSYSTEM (DTS) 

With the exception of the System Console, which has its own access port,
all of the asynchronous devices listed here can be connected to HP 3000
Series 900 computers via the Distributed Terminal Subsystem (DTS). The
subsystem consists of the following components:

 *  One or more Distributed Terminal Controllers.

 *  LAN Interface Controller (LANIC) Card.

 *  LAN hardware.

 *  Host software that controls DTS hardware.

HP 2345A Distributed Terminal Controller 

The HP 2345A Distributed Terminal Controller (DTC) provides the
connection between asynchronous devices and the HP 3000 Series 900
computers.  The DTC is housed in its own box, outside of the system
cabinet.  This alleviates system limitations, which are based solely on
cabinet space considerations, and allows flexibility in the layout of
terminal cables.  The figure labeled "HP 2345A Distributed Terminal
Controllers" represents the location of the DTCs in relation to the
system, the LAN, and the terminals/printers connected to the system via
the DTCs.


          HP 2345A Distributed Terminal Controllers

Each HP 2345A DTC can provide connections for up to 48 local devices or
36 remote devices using modems.  Its design is modular, with each DTC
providing six slots, each of which can contain one of the following
connector cards:

 *  Eight 3-pin RS-232-C local connections

 *  Eight 5-pin RS-422 local connections

 *  Six 25-pin RS-232-C modem connections

The DTC is controlled by software that is downloaded by its host system
when the DTC boot-up occurs.  The DTC boot-up is independent of the
system boot-up.  The downloaded software includes both operating code and
configuration information.  Once the host software has been downloaded,
the DTC is ready for operation.  Devices physically connected to its
ports and properly configured can then establish communication links with
the host computer.  More information on the download process and on DTC
operation can be found in Chapter 7 of the Asynchronous Serial
Communications System Administrator's Reference Manual (P/N 32022-90001).

An online diagnostic program, TermDSM, is available to help diagnose any
problems that may occur during DTC operation.  Use of the online
diagnostics is described in detail in the Asynchronous Serial
Communications Troubleshooting Manual (P/N 32022-90004).  A Power-On
Self-Test and an offline self diagnostic program are available to
diagnose any power-up problems.  Refer to the DTC Installation and
Service Manual (P/N 02345-90001) for more information on these tests.

Local Area Network 

HP 2345A Distributed Terminal Controllers are connected to the system via
a Local Area Network (LAN) connection, over coaxial cable that conforms
to the IEEE 802.3 Standard.  Hewlett-Packard provides two versions of
this cable, ThickLAN and ThinLAN, each of which provides certain specific
advantages to the user.

Each DTC has a unique LAN address, which is made known to the system
during system configuration.  Also referred to as the node address or the
station address, it serves to identify the DTC as a node on the LAN. You
should keep in mind, however, that while DTCs are treated as nodes for
purposes of configuration, they are not true nodes in the sense that we
normally consider nodes to be defined, since DTCs are not self-contained
computer systems.

ThickLAN Connection 

The thick version of the LAN cable supports up to 100 nodes.  Each node
on the LAN is connected via a Medium Access Unit (MAU). An Attachment
Unit Interface (AUI) cable runs between each node and its MAU, as well as
between the host computer and its MAU. A ThickLAN configuration is
depicted in the figure labeled "ThickLAN Configuration."


          ThickLAN Configuration

ThinLAN Connection 

ThinLAN is available in connection lengths of up to 185 meters (555 ft.)
and provides a more economical solution where DTCs are located in close
proximity to their associated computer system.  ThinLAN also provides
more flexible cable, which makes it easier to install, and allows it to
be bent around objects to accommodate its use in confined areas.  The
figure labeled "ThinLAN Configuration" depicts a configuration using
ThinLAN. Notice that the MAUs are replaced by a ThinMAU, which attaches
the LAN cable to the host computer.


          ThinLAN Configuration

More information on cabling and connection options can be found
in Chapter 6 of the Asynchronous Serial Communications System
Administrator's Reference Manual (P/N 32022-90001).

LAN Interface Controller (LANIC) Card 

Every computer in the HP 3000 Series 900 family of computers includes a
LAN Interface Controller (LANIC) card as part of its basic hardware.
This card is located in the System Processing Unit (SPU). It enables the
system to communicate with asynchronous devices via the LAN by providing
a basic connection between the computer and the LAN.

SOFTWARE 

The Asynchronous Serial Communications software provides the link between
the IEEE 802.3 LAN and the MPE XL system.

Control 

The software is included with the MPE XL Fundamental Operating System
(FOS).

MPE XL system software mirrors its related hardware in the sense that it
is structured in modules, with each module handling the control of a
specific type of hardware device.  These modules, or managers, know which
devices of their type are configured and handle all functions related to
those devices.

The ASC managers control terminal and printer I/O, as well as the
downloading and uploading of information for the DTCs connected to the
system.

The ASC managers determine the characteristics of the devices they
control.  They determine, among other things, the type of communications
protocol being used, what type of parity to use or whether parity is used
at all, and many other characteristics, which are explained in Chapter 3
of the Asynchronous Serial Communications System Administrator's
Reference Manual (P/N 32022-90001).

The ASC software also detects errors that take place during
communications and reports them to the MPE XL system.

Configuration 

The MPE XL system keeps track of all devices connected to it by means of
configuration files.  The network configuration information necessary to
initialize and control the Distributed Terminal Subsystem (DTS) resides
in a special configuration file.  This file is created through use of the
Node Management Configuration Manager (NMMGR) utility.  Chapters
9 through 13 of the Asynchronous Serial Communications System
Administrator's Reference Manual (P/N 32022-90001) provide a detailed
discussion of the configuration process.

MPE/iX Communicators