Path: utzoo!attcan!uunet!portal!cup.portal.com!mhyman
From: mhyman@cup.portal.com
Newsgroups: comp.dcom.modems
Subject: Re: Break definition
Message-ID: <8977@cup.portal.com>
Date: 11 Sep 88 17:49:50 GMT
References: <402@ucrmath.UUCP> <6095@galbp.LBP.HARRIS.COM> <569@pcrat.UUCP>
Distribution: na
Organization: The Portal System (TM)
Lines: 40
XPortal-User-Id: 1.1001.2549

In message <569@pcrat.UUCP> rick@pcrat.UUCP (Rick Richardson) asks:
> Where did you find the citation that a break is at least 20 bit times?
> The only citation I could find (in some obscure CCITT spec) defined
> break as the space condition for at least 130 milliseconds.  There
> was no formula based on the bps that the interface was operating at.

First some background:

  The bit (not baud) rate between the DTE side and the TELCO side of a
modem may differ.  Using V.22bis (standard 2400 bps modem) as an example:
The spec calls for an TELCO bit rate of 2400 b/s +/- 0.01%.  The DTE side
is allowed the range of 2400 b/s +1.0%, -2.5%. [+2.3%, -2.5% in modems with
``extend signalling rate range (optional)'' implemented].

  To handle the possible overspeed on input a modem is allowed to delete
stop bits now and again (no more than one stop bit deleted in every 8
characters sent [4 characters with extended option]).  When the receiving
modem receives a character with a missing stop bit from the TELCO it adds
a stop bit to the DTE.  It makes time for the added stop bit by making it
and the next 7 stop bits 12.5% shorter than a standard bit.  [25% shorter
for the next 4 stop bits with the extended option].

  What does this have to do with a break?  A break must be ``2M + 3'' bits
long at the receiver.  This requirement allows the receiver to tell the
difference between a break and consecutive NULL characters, the first with
a deleted stop bit.  Section 4.1.3 of V.22bis says:
  
        If the converter detects M to 2M + 3 bits all of ``start''
        polarity, where M is the number of bits per character in the
        selected format, the converter shall transmit 2M + 3 bits of
        ``start'' polarity.

M is typically 10 in async mode and 8 in sync mode.
 

--Marc
 
Marco S. Hyman          ...!sun!portal!cup.portal.com!mhyman
			mhyman@cup.portal.com