NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/dcom/modems-part1/2185

Internet Message Format | 1996-08-05 | 2.0 KB

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.dcom.modems
Subject: Re: Astounding high CPS rate!
Date: Sat, 20 Jan 96 22:05:59 GMT
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In article <[e-mail]>
           [e-mail] "Clifton T. Sharp" writes:

>Some time ago I read somewhere that there was supposed to be a "guard
>time" of two bit times between the stop bit of one character and the
>start bit of the next. This would suggest that one would have to
>divide by 12 (8+1+1+2). However, real-world performance doesn't
>suggest anything like this. Anyone ever hear of this "guard time"?

Not in those terms. It used to be the case that old devices weren't always
reliable receiving just 1 stop bit (possibly through poorer sampling methods,
possibly because the timing was less accurate). The normal approach to this
was to use 1.5 or 2 stop bits. All stop bits are are an interval where the
state of the line is 'mark' (i.e. as if it were transmitting 1 bits) So this
equates to a minimum time of 1, 1.5 or 2 bits worth of 'mark' after the
last data/parity bit and before the transition to the next start bit. Any
remotely modern UART can cope quite happily with just 1 stop bit so cps
can really be up to bps/10 for 10 bit character frames such as 8-N-1. As a
legacy of old UART compatibility you can program PC serial ports to support
2 stop bits (don't know about 1.5) but these days there's no reason to.

--
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Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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