HP 3000 Manuals HP 3000 Manuals
How to Control Terminals
DTS helps control the operation of terminals. It automatically handles
the input and output of data to the terminal as explained earlier in this
chapter under "How to Control the Flow of Data." Alternative methods for
controlling terminals are as follows:
* Echoing characters to the terminal screen as they are typed at the
keyboard.
* Allowing special characters to be processed as intended. For
example, [Enter] signals the end of input data and the backspace
character causes an unwanted character to be deleted from input
data. Other examples of special characters include system break
([Break]) and subsystem break (usually [CTRL]-Y).
* Modifying the transmission speed through the SPEED or SET command.
It is possible to programmatically change the method that the DTS uses to
control a terminal. A program can change the following attributes:
* Parity error checking.
* The method of input for a read from character mode to block mode.
* The set of special characters recognized by the DTC.
* The maximum byte count or a maximum time to enter data.
Each of the methods for controlling terminals is explained in the
following pages. Refer to the Asynchronous Serial Communications
Programmer's Reference Manual for more information on programmatic device
control.
Echo
When characters are typed on a keyboard the terminal user expects each
character to appear on the screen as it is typed. This is referred to as
echo. Any of the following settings determine whether echo occurs:
* Local echo enabled.
* The modem echo is enabled.
* DTC echo is enabled.
* Typeahead echo is enabled.
Only one of these settings should be enabled at any time. Each of these
settings are described below. Hewlett-Packard recommends setting DTC
echo.
(DTC echo is automatically enabled for devices configured as terminals.)
Local Echo.
When terminals are installed, local echo is turned off. If local echo is
turned on, the terminal will echo each character to the screen as it is
typed. Local echo is used with half-duplex communications lines and some
statistical line multiplexers. Note that block mode applications usually
turn local echo on because input characters are sent to the DTC in a
block of data, instead of one character at a time.
Modem Echo.
Modems are able to echo characters as they are typed. This feature
should not be used. This feature is usually disabled in the modem's
default configuration setting.
DTC Echo.
DTC echo is enabled for both terminal type file 10 and terminal type file
18. The DTC will echo to the terminal screen each character as it is
typed. DTC echo provides a simple form of data protection since data
that appears after it is typed has been successfully transmitted to the
DTC and back to the terminal screen again. Disable DTC echo for block
mode reads; Local echo takes over the function of DTC echo because data
is transferred to the DTC in a block of characters, instead of one
character at a time.
Typeahead Echo.
Typeaheadallows the terminal user to continuously enter data without
having to wait for the system to process the data and return the MPE
prompt (:). Typeahead echo mode determines whether input characters will
be echoed to the terminal screen once or twice.
Disabling Echo.
Sometimes characters should not be echoed to the screen, for instance,
when entering a password. In this case, the computer disables echo.
However, if characters should not be echoed to the screen at other times,
the SET ECHO=OFF command
can be used. Echo can also be disabled by a program through the use of
the FCONTROL or FDEVICECONTROL intrinsic. Note that programs controlling
block mode reads must disable DTC echo.
Special Characters
Special characters are ASCII characters that have certain functions
assigned to them; they signal the DTC to take a particular action. The
results of using special characters are called special control functions.
These include the characters described below, as well as the characters
used for protocols. The characters described below are defaults and are
modifiable by applications.
Backspace.
When a backspace characteris entered, the DTC deletes the previous
character from the input data and echoes the backspace character to the
terminal (provided echo is enabled). When the echoed backspace character
is received, the cursor on the screen moves back one character and is
positioned at the character that was deleted. Use [Backspace] or
[CTRL]-H to backspace.
Line Delete.
To delete a line of data while in character mode, press [CTRL]-X. After
[CTRL]-X is pressed, three exclamation points (!!!), along with a
carriage return and line-feed, are displayed, signaling that all read
data was deleted.
End-of-Record Character.
When a terminal operates in character mode, a special character is set to
enable the user to end a read. This is called the End-of-Record (EOR)
character.
It is also called a record terminator or line terminator. Two types of
EOR characters exist: standard EOR character and additional EOR
character.
Standard EOR Character
When users finish typing a line and press [Return] (the standard EOR
character), data is transmitted. The carriage return character
terminates the read but it is not included in the data of the read or
counted in the number of actual characters read. When the EOR character
is detected, a carriage return is echoed to the screen and the ASC
software generates a line feed. This places the cursor at the beginning
of the next line. The EOR character can be replaced with another
character during transparent editing; this is done through the FCONTROL
41 intrinsic.
Additional EOR Character
Also referred to as an alternate EOR character, this character is
included in the data and the actual character count (byte count) of the
data. The read does not end normally, but terminates in an error stating
that the additional EOR character was encountered. The program that
initiated the read must recover from the error by deleting the additional
EOR character from the input data, subtracting one from the byte count,
and sending a carriage return and line feed to place the cursor in its
proper place.
AEOR characters can be defined through the FCONTROL 25 FDEVICECONTROL
192, 40
Note that when a read terminates, the program must call the FCHECK
intrinsicto see why the read terminated. If FCHECK returns an error
code of 31, it means the read ended with the additional EOR character.
Up to 16 AEOR's are available but only through the use of FDEVICECONTROL
192, 66.
System Console Attention Character.
When [CTRL]-A is entered on the system console,it signals a [CTRL]-A
console command. The computer sends the system console a carriage return
and line feed along with an equals sign prompt (=), signaling that it is
ready for the command. [CTRL]-A commands are allowed only on the system
console. For more information on the system console and its special
commands, refer to MPE/iX Managing Peripherals (32650-90031).
NOTE [CTRL]-A is not accepted as a special character from a terminal
that is connected to a DTC.
System Break.
Using default operating conditions, the ASC softwareresponds to a system
break signal
from the terminal and alerts the computer. The system break function
interrupts the execution of programs, subsystems, and most MPE/iX
commands. Once the program or subsystem is interrupted, it can be
aborted by entering the ABORT commandor the program or subsystem can be
restarted by entering the RESUME command.
To transmit a system break signal, press [Break]. System break can be
disabled programmatically. Refer to the MPE/iX Intrinsics Reference
Manual (32650-90031) for more information on system break.
Some application programs, such as block mode programs,change the
settings of terminalsand the characteristics of their device files.
These programs should return the devices to normal operating mode before
they complete.
If a program does not disable break (through the FCONTROL or
FDEVICECONTROL intrinsic), the user can still use [Break]. If [Break] is
pressed, the program might not be able to recover but the MPE/iX
operating system will by assuming that the terminal is in normal
operating mode.
A VPLUS block mode application can be reset by pressing [CTRL], [Shift],
and [Reset] simultaneously. Other block mode applications are harder to
reset. Sometimes, it is necessary to turn the terminal off and on.
Other times, the port must be reset. Refer to Troubleshooting Terminal,
Printer, and Serial Device Connections for information on resetting ports
if host-based management is being used. Refer to Using OpenView DTC
Manager for more information on resetting ports if PC-based management is
being used.
Subsystem Break.
Subsystem breakstops a program-local or subsystem-local command and
enables the user to enter a different command. The default subsystem
break character in standard editing mode is [CTRL]-Y.
Subsystem break is not enabled by default. It must be enabled
programmatically through the FCONTROL 17 intrinsic.A subsystem break
character can also be defined when entering transparent editing through
the FCONTROL 41 intrinsic.
Transmission Speed
Asynchronous data transmission speedson 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 connected devices on DTC 16iX/16MX and DTC 72MX.
The speed chosen
must be supported by the terminal. A terminal can be changed to any
supported speed if it is directly connected to the DTC. The speed of a
terminal connected to a modemshould be set to the speed of the modem and
the telephone line being used.
Parity
Parity verifies that each character of data is transmitted between a
device and the system without error. It detects data transmission
errors, but does not correct them.
Parity can be used only with 7-bit character sets,such as USASCII,because
the eighth bit is used as a parity bit. Character sets that use all
eight data bits cannot use parity. Examples include the terminal's
alternate character setand character sets used for European languages.
Two types of true parity exist: odd and even. Odd paritycounts the
number of ones in the seven data bits for each character. Every
character has an odd number of bits set to one. If there is an odd
number of ones, the eighth bit (parity bit) is set to zero; if there is
an even number of ones, the eighth bit is set to one. When the character
is transmitted, the receiver verifies that the number of bits set to one
is an odd number.
If one of the bits is transmitted incorrectly, the number of bits is even
and an error is detected. If two bits are transmitted incorrectly, the
character might contain an odd number of ones and the error won't be
detected. Since most data transmission errorsinvolve only one bit of the
character, most transmission errors are detected.
Even parityworks the same way as odd parity, except the total number of
bits set to one in each character is set to an even number.
Hewlett-Packard asynchronous devicesand the ASC softwareuse two other
parity settings as well. These are called ones and zeros. With ones
parity,the parity bitof each character is set to one; with zeros parity,
the parity bit is set to zero. If the eighth bit is involved in a
transmission error, it is detected because the bit is not what was
expected.
It is possible to not use parity at all. This is called none parityor
8-bit pass-through.Neither the sender nor receiver of data sets the
eighth bit or checks it for parity. (A terminal using a 7-bit character
set and no parity places a zero in the eighth position so that characters
being transmitted are always eight bits long.)
Parity can be enabled through the FCONTROL intrinsic or the
FDEVICECONTROL intrinsic.When enabled, the ASC software generates parity
on outgoing data and checks for parity errorson incoming data. After the
parity is checked, the parity bit is set to zero because the program
using the data has no need for parity information. Parity checking is
handled the same way in block modeas in character mode.
Parity settings for terminalsmust match the type of parity used by the
ASC software.The default parity setting for HP terminals is to generate
zeros parity, but not to check parity on data from the computer. HP
terminals will work correctly with the ASC softwarewithout having to
modify any configuration
values.
If enabling parity, verify that the program which uses the FCONTROLor the
FDEVICECONTROL intrinsiceither requests that the user change the
terminal's parity setting to the new setting of the ASC software or pass
the proper escape sequences to change the terminal's settings.When a
parity error is detected, the read is completed in error. The ASC
software reports the error to the program reading the data.
Input Modes
Data can be input in character modeor block mode. Characteristics and
limitations for each are determined by the ASC software and the attached
terminal.
Character Mode.
Characters are transmitted to the DTC as they are typed. The DTC expects
to receive them this way. If DTC echo is enabled, characters are echoed
back to the screen as they are received.
Character modeis how terminals are opened, whether by a sessionor a
program. Character mode reads are terminated with the carriage return
character
([Return] or [CTRL]-M).
Block Mode.
When a terminal operates in block mode, characters are held in the
terminal's memory (buffer)as they are typed. Characters are not
transmitted to the DTC until a specific action is taken. Pressing
[Enter] at the end of a block of data will normally send it to the DTC.
An application program can use an alternate method to end a block mode
read.
Refer to "Terminating Reads" later in this chapter for more information
on ways to end a block mode read.
Block mode is enabled programmatically by executing a block mode
applicationat the terminal. The program needs to change the terminal's
configuration settings. It should do this automatically by sending
escape sequencesto the terminal.
Blocks of characters can be input to the DTC a line at a time (called
line block mode)or a page at a time (called page block mode), depending
on the type of block mode used. Because characters are displayed on the
screen as they are entered, local editing and cursor control features can
be used to alter a line or page before it is transmitted to the DTC.
Data is transmitted one character (literally one byte) at a time, with
each character bound by start bitsand stop bits.Block mode sends data in
one transmission "burst," although the characters are transmitted one by
one in a long stream.
Data Editing Modes
Data input from a terminal is usually intended as data for the computer:
commands, input to a program, etc. Some characters are not interpreted
as data, but as special characters.When these characters are encountered,
the DTC or the ASC softwareis signaled to perform a specific function.
The functions vary from deleting a character to interrupting a program or
subsystem.
The data editing mode defines which characters are recognized as special
characters by the system. The types of data editing modes are as
follows:
* Standard editing.
* Transparent editing.
* Binary editing.
Special character setsare defined for each editing mode. All special
characters apply in standard editing mode,fewer apply in transparent
editing mode,and no special charactersare recognized in binary editing
mode.Refer to the Asynchronous Serial Communications Programmer's
Reference Manual for more information on data editing modes.
Terminating Reads
Reads can be programmatically terminated in one of several ways:
* An EOR or AEOR characteris sent from the terminal.
* The input byte count is reached.
* The read limit timer
expires.
* The block moderead timer expires.
* The system break character(if it is enabled) is sent from the
terminal.
* The subsystem break characteris sent from the terminal.
* The [CTRL]-A character is sent from the system console.
* The DTCencounters a parity error
(only when parity checking is enabled).
Note that when using binary editing mode,a read cannot be terminated by
any character. Binary reads can be terminated only if either:
* The input byte count is reached.
* The read limit timer
expires.