[GreenKeys] Newbie q: running open

Sun Jun 28 12:52:01 EDT 2015

Wayne's definitely an expert and my experience has been the same.  I think
it's important to note a difference between an open loop situation and
a TU (or modem) listening to noise without proper demod of an AFSK signal.

When the loop is open--  ie, no current is flowing-- the TTY receives
a constant space level which is equivalent to perpetually receiving
the NULL character.  My machines do all of the operations to print a
character EXCEPT hammering it and advancing the carriage when the loop
is open.  This is because they constantly see a start bit which causes the
machine to begin reception of a character, then it sees only space level
(zero bits) for the code of that character and finally it sees the stop
bits since the line is still at space level.   Then, as Tony mentioned,
the machine instantly sees another start bit and begins the process all
over again without any intra-character rest period.

Comparing this to a TU or modem listening to no tones or to noise with the
loop still functioning means you have basically random characters being
shifted into the machine as the modem occasionally toggles between mark
and space, clocking out all sorts of weird bit patterns that inevitably
cause the TTY to see a start bit, clock in some random bits and then
print the random character represented.  In my experience, a modem that
isn't listening to decent FSK from the other end tends to spend a far
greater percentage clocking out space level than mark which I think gives
the appearance of open loop behavior intermixed with occasional random
characters.  But-- I would say this is all a function of the modem design,
whether it has some form of carrier detect/squelch mechanism, etc. to
keep the random junk from coming out when it isn't properly demodulating.

Chris NØJCF

On Sunday (06/28/2015 at 03:19PM +0000), teletypeparts at comcast.net wrote:
> My experience with 32 and 33's is that when running open, nothing is printed unless the running open is intermittently closed or the blank function bar is missing or not working.  I have a 32 that prints the style of the typewheel when it runs open, but that is not normal.  
>   
> Sometimes a 33 will print the left arrow when its running open.  Again, that is the exception. 
>   
> Normally nothing is printed when running open and the 32 or 33 sounds like its going a little faster and that's because it is.  No double bit stop pulse is received. 
>   
> Wayne 
> KB1FDW 
> 
> ----- Original Message -----
> 
> From: hwhall at compuserve.com 
> To: dave at horsfall.org, greenkeys at mailman.qth.net 
> Sent: Sunday, June 28, 2015 12:42:53 AM 
> Subject: Re: [GreenKeys] Newbie q: running open 
> 
> > but does it print random gibberish, or just make a racket? 
> 
> Both. Constant racket, and intermittent random characters. 
> 
> Wayne 
> WB4OGM 
> 
> -----Original Message----- 
> From: Dave Horsfall <dave at horsfall.org> 
> To: Greenkeys <greenkeys at mailman.qth.net> 
> Sent: Sat, Jun 27, 2015 9:58 pm 
> Subject: [GreenKeys] Newbie q: running open 
> 
> OK, my only RTTY experience has been of the computer variety, but I'd love 
> to
> learn more about these beasts.
> 
> A term I've seen often is "running open"; I
> understand that it happens 
> when there is no signal present (?), but does it
> print random gibberish, 
> or just make a racket?
> 
> Thanks.
> 
> -- 
> Dave
> Horsfall  (VK2KFU)  "Those who don't understand security will
> suffer" http://www.horsfall.org/ It's just a silly little web site, that's
> all...
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