vt220 question

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue May 4 18:25:05 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

In a sense it is...

On a current loop system you have 2 states. Current flowing in the loop
(the idle sate), current not flowing in the loop. A break is sent by
interrupting the loop (current not flowing) for a suitable period (200ms
is common).

On an RS232 device, the 2 states are voltages on the transmit data pin. A
-ve voltage corresponds to the current-flowing state of the current loop,
a +ve voltage corresponds to no current flowing in the current loop. The
idle state is the -ve voltage, of course. A break consists of making the
transmit data pin +ve for 200ms (or whatever).

> 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

It is not a code in the sense that 8 bits are sent with start and stop
bits, etc. In fact the normal way to detect a break at the receiving end
on a modern system is to look for a framing error detected by the UART
chip -- this means the received character didn't have valid stop bits,
which is the case for a true line break.

-tony
Received on Tue May 04 2004 - 18:25:05 BST