Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)

Chapter 3 61

Common Device Control Functions

Reading From Asynchronous Devices

Block mode is enabled programmatically by a block mode application

when it is executed at a terminal. The terminal’s conﬁguration settings

need to be changed also, in order for the terminal to function properly

in block mode. This can be done by prompting the user to change the

terminal’s conﬁguration settings or by programmatically sending

escape sequences to the terminal.

Block mode allows a terminal user to see data on the screen as it is

being typed in and to use the terminal’s local editing and cursor control

features to alter the data before it is transmitted to the DTC. It is

available only on terminals that have block mode capabilities; all of the

terminals supported for use on MPE/iX systems are capable of handling

block mode data transmission. When using a PC, however, you should

verify that the terminal emulation program you are using is capable of

handling block mode transmission.

User block mode can be invoked through a call to FCONTROL(29) and

makes it possible for your program to control the way in which a block

mode transaction will take place. See the discussion of

FCONTROL(28,29) in chapter 8 for more information on user block

mode.

NOTE

A convenient way to handle block mode processing is provided by the

VPLUS intrinsics.VPLUS intrinsics automatically perform many of the

block mode terminal control operations that you are otherwise

responsible for. It is recommended that you use this method whenever

possible. See the VPLUS Reference Manual for more information on

using the VPLUS intrinsics.

Field Mode

Enhancements have been added to provide the performance of block

mode and the ﬂexibility of character mode, called ﬁeld mode.

Field mode was created to help application programmers control

certain keys on the keyboard. Some applications require certain keys to

be controlled by the application and not processed locally in the

terminal. Block mode applications do not recognize any of these special

keys except in some cases where they are processed locally by the

terminal. To process the special characters would require reading

characters one at a time. This would result in performance degradation.

Field mode allows the users to collect normal user data input without

having to interrupt the CPU for each character, yet still have many

ways to return control to the application when the user enters

command input. The basic building blocks for ﬁeld mode are

summarized:

• Inter-byte timer. Provides a mechanism by which the DTC can

terminate a read and forward data to the host. It is useful for

collecting data where there is no guarantee that an end-of-record or