
From Paul.Williams@rrds.co.uk Sun Jan 10 23:27:55 1999
Date: Thu, 07 Jan 1999 15:30:27 +0000
From: Paul Williams <Paul.Williams@rrds.co.uk>
To: Richard Shuford <shuford@cs.utk.edu>
Subject: ansi_dec_controls_news.txt

Dear Richard,

Happy New Year.

At the moment I am working on two documents for my terminals web site
<URL:http://www.celigne.co.uk/terminal/>. They will explain:

1. How the standards ECMA-35 (character code structure) and ECMA-48
   (control functions) relate to Digital's VT series of terminals.

2. How Digital has made use of the private extension facilities (eg.
   using ESC 7 and 8 to save and restore the cursor).

I've been using Dejanews and your site to investigate how broad a
coverage to give these documents, ie. to see what questions people have
asked in the past.

I believe there are two mistakes in ansi_dec_controls_news.txt that
you might wish to correct, even though I'm several years late in
joining the thread of conversation!

________________________________________________________________
1. Private extensions to parameter strings for control sequences

A post from James Carlson (about line 1020) explains:

:  "CSI ? 2 ; 4 h" is equivalent to "CSI ? 2 h CSI ? 4 h". The flags
:  (like the '?' character) are global within a single CSI sequence.

and then,

:  the '?' flag can appear anywhere. For example, this is a sequence
:  equivalent to the one above, even if it is a bit strange looking:
:
:     CSI 2 ; ? 4 h

Although his first example is correct, the second example is not
allowed by ECMA-48, which says that the use of '?' in any position
other the first character of a parameter string is reserved for
future standardisation. The same rule applies to the other "private
flag" characters, '<', '=' and '>'.

Of course, standards change over time and if terminals actually allowed
the second example then we might just add the caveat that this
construct is not recommended even though you can get away with it.

However, I've tried this sequence on a VT320 and a VT420 and it doesn't
perform any changes to either ANSI or DEC private modes.

I was pleased about this because I have found the VT320 to be very
compatible with ECMA-48 so far, although I appreciate that the behaviour
of  terminals in error conditions is not specified by this standard and
could perhaps include partial execution of this erroneous sequence.

______
2. ECH

On line 1170 there is a small typo where it says that the Erase
Character (ECH) sequence is invoked by the codes:

   Esc yy X

which should say "Esc [ yy X". In fact this paragraph is almost
identical to the one around line 780, so could perhaps be removed
or made identical to that one.

_______
Trailer

Richard, do you get lots of email from people asking you for
specifications of terminals? I was just wondering, because I've started
receiving more mail now, and my access logs show that many people arrive
at my site by typing "vt100" into Altavista or Yahoo.

I even had an email from one person who wanted me to tell him how to
write a terminal emulator! I wrote back suggesting that he shouldn't.

Are you still at UT?

Regards,
Paul
-- 
Paul Williams               Racal Radar Defence Systems Ltd,
Principal Engineer          Manor Royal, Crawley, West Sussex, RH10 2PZ.
Tel: +44 (0)1293 644540     Fax: +44 (0)1293 542812
