On Mon, Jan 14, 2002 at 07:21:11AM -0600, Stephen M Jones wrote:

> In a previous message
> You suggested something interesting.  Setting the port speed on the terminal
> server to 300bps, connecting to it sending some characters and then setting
> it back to 9600 and connecting back to see if you're at the P00>> prompt.
Yes. A break is nothing else as forceing the serial line for a "long"
period to be in the "active" state.  Depending on the baud rate you 
can calculate the max. time that a char needs to be transmitted. If 
the serial line is not in the "inactive" state after the transmission 
of that char for at least a singe bit period (=stop bit), you get a 
framing error on the UART.
As you have 10 bits per char (1 start, 8 data and 1 stop bit = 10) 
you need a baud rate that is max. 1/10 of the normal baud rate (9600).
With this low baud rate a single bit needs more than the whole char 
in the normal baud rate. Thus framing errors will be guaranteed. :-)
In real live it may be necessary to generate a break that is 
much longer than a single char. I can remember that there was 
a spec about that, but my "RAM refresh" is a bit ... You know? ;-)
So going to 300bps is quite save as the ratio is 9600/300=32 and than
you can get more time by using the right char like "p" =0111 0000
That will have at least three bits in sequence of the same state.
(Don't know at the moment if RS232 is active high or low.)
So you get a ratio of 32*3=86 from single char at high speed to 
shortest period in low speed. This should be enough. 

Or connect a power switch box with serial interface to your terminal
server. ;-)

> Have you had much success with that? ;-)  
No, as i don't need it. My terminal servers support break. 
This method was found by a friend. (Hi Michael.)
-- 



tschüß,
         Jochen

Homepage: http://www.unixag-kl.fh-kl.de/~jkunz/