🚀 go-pugleaf

bbs@NCoast.ORG (XBBS System) writes:

> Could someone out there explain to me why a signal can physically travel
> further on a metal media at a slower baud rate ?

Ok.  First, computer signals are a square wave, typically a negative
voltage for a one, positive voltage for a zero.  The faster the baud
rate, the shorter the "wavelength".

Now a square wave isn't really a wave at all, but can be thought of as
the sum of a wave and all the odd harmonics, ie third, fifth, seventh
and so-on (the second harmonic is twice the frequency of the primary,
he third harmonic is three times the primary etc).

The interesting thing about this model is that the "corners" of the
square wave are made up of the higher frequency harmonics; the cleaner
the corner, the higher the frequency of the harmonics that make it up.
Conversely, the lower the frequency of the highest harmonics, the
rounder the corner.

The power of a harmonic is proportional to its order, ie the third
harmonic is a third of the power (???, help me out guys!) of the primary.
As the order of the harmonic goes up, the power goes down, and makes the
wave more likely to be squashed by the attenuation of the line.

Now, by the time the signal gets from one end of the wire to the other,
it can be quite rounded.  A single bit at 9600 baud might look like:

                    TX end                      RX end

       +6V -      ..........                     ....
                  .        .                   ..    ..
        0V -      .        .                  .        .
                  .        .                ..          ..
       -6V -.......        .......      ....              ....

                  | 1/9600s|                  | 1/9600s|

That contains a recognisable bit, but at 19200, with the same level of
attenuation, our bit is going to look a bit like:


                     TX end                      RX end

       +6V -         ......
                     .    .                       ....
        0V -         .    .                      .    .
                     .    .                    ..      ..
       -6V -   .......    .......          ....          ....

                     |    |                      |    |
                    1/19200s                    1/19200s

Now, the signal hasn't finished rising to the +6V required to register
as a space (RS232 idles in the mark or 1 state, between -6V and -12V(?),
a space or 0 being a +6V to +12V signal) in the UART (although most
trigger well below the +/- 6V mark) before it starts diving back toward
the mark state.  Of course two zero bits will be recognisable (as at
least one) as they'll look just like a single bit at 9600 baud.

> I see this often when looking at specifications for line drivers, etc.
> And usually the slower the baud rate is, the further the distance can be.

Yes, although it always amazes me what you can get away with if you know
what you are doing ... I can show you cables that go well over 100m at
19200 baud through a factory, in violation of all the rules....  (the
cables are shielded and grounded, each data line is twisted with a signal
ground line to provide that extra bit of shielding, but otherwise it's
pure RS232).  It works, too. (I've also seen cables/equipment in a room
twenty yards from the host computer that couldn't hack it at 19200 though.)
Specifications always tell you what is guaranteed to work or your money back.

> Someone says it has something to do with the waves and frequency of the signa
> Another person suggested that the more powerful the signal, the attenuation
> it will have. I didn't quite understand until he compared it to how FM radio
> stations usually have more power but the signal doesn't travel as far to
> AM stations that may be heard by half the country.

Ah, now this is quite different territory.  It has to do with the fact
that radio wave propagation in the AM band (500 or so KHz to 1800 or so
KHz) is utterly and totally different to the FM band (~80MHz to ~110
MHz).  FM is mostly line-of-sight, whereas AM is mostly ground wave.
Borrow an ARRL handbook or something -- this is hard to explain without
a lot of hand-waving and glossing over important bits.

Hope that helps.

Don Stokes, ZL2TNM  /  /                             don@zl2tnm.gp.co.nz (home)
Systems Programmer /GP/ GP PRINT LIMITED  Wellington,       don@gp.co.nz (work)
__________________/  / ----------------   New_Zealand__________________________