🚀 go-pugleaf

(posted to comp.dcom.modems and mailed to "Alfredo Aletto"
<alalett@tin.it>)

On 28 Nov 1998 23:59:53 GMT, "Alfredo Aletto" <alalett@tin.it> wrote:

>Please, who can tell me if it's possible to generate a <Break-Signal>
>(signal that program on computer send to modem or signal between
> modem-modem) using a standard transmission ?

A break signal is something which could not possibly be mistaken for a data
byte.  It is implemented via a specific feature in the UART and not
triggered by any sequence of bytes.

>In other words what is in concrete a <Break-Signal> ?

As you may know, your RS-232 interface (be it physical or, as in the case
of most internal modems, virtual) is implemented as an asynchronous stream
of bits: to send a data byte at any time, the UART takes the line (which is
'parked' at a data value of '1') and sends a '0' for one bit time, followed
by your data bits, followed optionally by a parity bit, followed by a stop
bit of value '1'.  A break signal is a value of '0' sent for longer than
the time that it would normally take to get to the stop bit, essentially
causing a framing error at the other end of the RS-232 connection.  Most
modern UARTs (including the ones used in PCs) actually check for break
conditions (receive data line held low for an entire character time) and
report a break signal specifically rather than just report it as a framing
error.

>In many programs (see "Boyan", "Telemate", "PcPlus", etc.) a 
><Break-Signal> is transmitted with Hot-Keys, such as 
><Ctrl-End> or <Ctrl-B> .

These DOS programs have their own serial drivers (i.e., they access the
UART directly and sends a break by setting a specific bit in the UART's
line control register for a given period of time (usually a tenth of a
second, which is longer than the longest data word at 110 bps).

If you're programming in a more complicated environment (e.g. Windows),
you'll need to user an API to access communications functions, and that API
should define a way to request that a break be sent.

>In modems with chipset Rockwell it is associated with this
>standard "AT" commands: "AT\Bn" and "AT\Kn", where n is a decimal integer
>value.

There is no universal rule about what a modem should do when it receives a
break signal.  In the old days (jeez, you'd think that I had a mile-long
beard and walked with a cane) when modems were just modulators and
demodulators, they lacked the intelligence to do anything but just pass it
through.  Today you can tell the modem to interpret the signal many ways,
from causing it to throw away any queued outbound data to dropping to
command mode (like sending the escape sequence).

The AT\Bn command simply tells the modem to send a break signal through to
the other modem immediately... but that only works if you're in command
mode.

--
Geoffrey Welsh; crs0794@inforamp.net; http://home.inforamp.net/~crs0794/
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