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

Describing Asynchronous Devices
How to Control Terminals
Chapter 9
177
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 break stops 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 speeds on 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 modem should 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 set and character sets used for
European languages.
Two types of true parity exist: odd and even. Odd parity counts 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 errors
involve only one bit of the character, most transmission errors are detected.
Even parity works 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 devices and the ASC software use two other parity
settings as well. These are called ones and zeros. With ones parity, the parity bit of 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.