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Re: Which "baud"?

Quoting Johann Spies (jhspies@adept.co.za):
> I am confused about what is exactly meant by the instruction in the
> documentation of my UPS: "Port must be set to run at 2400 baud"

Yes, it would have been more sensible to write 2400 baud/bps,
as they're the same for a port.

> According to the Serial-HOWTO.gz it seems to me that even fast serial
> port run at 2400 baud:
>      baud
>         The baud rate is a measure of how many times per second a
>         signal, for instance one sent by a modem (modulator-demodulator)
>         changes. For example, a baud rate of 1200 implies one signal
>         change every 833 microseconds. Common modem baud rates are 50,
>         75, 110, 300, 600, 1200, and 2400. Most high speed modems run at
>         2400 baud. Because of the bandwidth limitations on voice-grade
>         phone lines, baud rates greater than 2400 are harder to achieve,
>         and only work under very pristine phone line quality.

(This is the slink HOWTO, which has been superceded, in case you
can't find this quotation, folks.)

As it says, this paragraph is about modems, not serial ports.
You need the next paragraph:

        The bps rate is a measure of how many bits per second are
        transmitted.  Common modem bps rates were formerly  50, 75, 110,
        300, 1200, 2400, 9600.  Today they are 14.4K, 28.8K, 33.6K, and
        56K (these do not correspond to the rates over the serial-port-
        to-modem cable which are in addition to the old modem bps rates
        (50-9600): 19.2K, 38.4K, 57.6K and 115.2K).  Using modems with
        V.42bis compression (max 4:1 compression), rates up to 115.2K
        bps are possible.  This is what most people mean when they
        misuse the word ``baud''.

In other words, the serial port is dealing with:

        (50-9600): 19.2K, 38.4K, 57.6K and 115.2K).

IIRC the typical relationship bps / baud used to be 4,
i.e. your 1200bps terminal was communicating over the phone
line at 300baud. Now that factor is higher.

> But when I do  stty -a -F /dev/ttyS1  I get
> speed 38400 baud; rows 0; columns 0; line = 0; etc.
>       ^^^^^

I think I warned you not to use the -F switch in my first posting
to you. If you need to convince yourself, try typing the line you
have typed above into a VC and an xterm. Then try and explain how
the serial port speed changed when you switched between the VC and
X! Alternatively, try running stty under strace and see if there's
any trace of ttyS1 in the output.

Now try replacing "-F" with "<" and you should get the right answer.

BTW when you said the script in rc.boot didn't work, that may be
the reason.


Email:  d.wright@open.ac.uk   Tel: +44 1908 653 739  Fax: +44 1908 655 151
Snail:  David Wright, Earth Science Dept., Milton Keynes, England, MK7 6AA
Disclaimer:   These addresses are only for reaching me, and do not signify
official stationery. Views expressed here are either my own or plagiarised.

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