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

194 Chapter9
Describing Asynchronous Devices
How to Control Terminals
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.