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

Chapter 9 195
Describing Asynchronous Devices
How to Control Terminals
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.
It is possible to not use parity at all. This is called none parity or 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 errors on 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 mode as in character mode.
Parity settings for terminals must 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 software without having to
modify any configuration values.
If enabling parity, verify that the program which uses the FCONTROL or
the FDEVICECONTROL intrinsic either 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 mode or 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 mode is how terminals are opened, whether by a session or a
program. Character mode reads are terminated with the carriage
return character (
[Return] or [CTRL]-M).