This article describes why trying to do block-mode terminal emulation on a
VAX running VMS is not a good idea. Essentially, it is a very complex,
time-consuming process to convert software.
Having block-mode terminal capabilities on a VAX running VMS requires more
than writing a terminal emulator. It also requires rewriting all of the
programs that run on the VAX so that they support block-mode terminals. At
the lowest level, VMS does support block-mode terminals. However, the
lowest level only lets these features be implemented by an application.
This requires the VMS application, itself, to implement all of the
block-mode input and output routines that are usually accessible from the
standard I/O routines on machines that run block-mode terminals.
Block-mode transfer is available from a specific model of DEC terminal in
the VT100 family, the VT131. Currently, no terminal emulation package is
available for the Macintosh that implements the block-mode features of the
VT131. The alternative, then, is to write a terminal emulator with the
necessary features.
Unfortunately, writing a terminal emulator that has block-mode features
does not solve the problem on a VMS machine. The following quote from
Digital's "Terminal and Printers Handbook" explains this:
"Digital currently supports the VT131 block mode under VAX/VMS version 3.0
and later versions. Support is at the operating system level, and this
requires the use of third-party application programs to take full advantage
of the VT131 features. This allows Digital software OEMs that use block
mode terminals to use their software on Digital hardware with minimal
modification. Digital has no plans to provide application software for the
VT131 in other than VT102 (conversational) mode."
What this means is that although you could write a terminal emulator that
has block-mode features, none of the standard VMS application software
would take advantage of those features. They would read the terminal in
character-by-character (conversational) mode, anyway. Thus, to implement
block mode, you would have to write the terminal emulator and rewrite every
application on the system to support block mode. For example, the VMS
editor would need to be completely rewritten.
The use of AppleTalk VMS does not solve this problem for the same reason
that writing the terminal emulator will not. Standard VMS applications are
still written for conversational mode. It is possible that performance
might degrade in a situation where each character is transmitted in its own
AppleTalk packet with the echoed character also returned in its own packet.