vt220 question

From: John Lawson <jpl15_at_panix.com>
Date: Tue May 4 10:22:12 2004

On Tue, 4 May 2004, Jay West wrote:

> Ok, the post from Kevin reminds me of a VT220 question I've been wondering
> for some time. Is there a "BREAK" key on the vt220 terminal???? I can't find
> it, nothing is labelled "break". I'm not looking for a key combo that
> interrupts a specific host platform like ^C or ^P or something, I want to
> generate a real electrical "BREAK" signal via the keyboard. What is it?

  Well, two things. First, a 'real electrical "BREAK" signal' is not a
signal per se, but a momentary interruption in the actual signal current
flowing in the terminal circuit. This makes sense in a 'current loop'
environment, such as DC-loop teletype circuits. One presses the 'BREAK'
key, switching off the loop, and all the machines in that loop begin to
'run open' - they just start cycling mechanically, since in most classic
mechanical TTYs, the loop current serves to hold the mechanism clutch(es)
in the 'start' or 'ready' position, waiting for code pulses to come along
and set up a character printing cycle.

 Second:

  I am not sure that the newer 20ma current loop devices - ASR33s, and DEC
(and other) devices that use 20ma loop serial - will use this actual
electrical 'break' for an "ATTENTION" signal. Anyone...?

  On my VT220, as connected to a VAXStation 3100, depressing the 'BREAK'
key on the LK401 causes an immediate processor halt and the display of a
monitor prompt... >>> . Since I have it hooked up using the RS232
port, and since there is no such thing as a real interrupting 'break' in
RS232 - it must be some other code... or, is it? The VT220 Programmer
Pocket Guide just says that he first 5 top-row keys are "local function
keys and do not generate codes."

  The actual VT220 Operator's Manual expands on this a bit - the 'BREAK'
function has to be enabled in set-up, and one can also issue a shift-break
to start a 'disconnect', or a cntrl-break to send the 'answerback'
sequence to the host machine.

  But it still doesn't tell me what the hardware does when pressing
'BREAK'. IF no one knows, I'll have to get out the break-out box and
see if I can trap it.

  Cheers

John
Received on Tue May 04 2004 - 10:22:12 BST