Sorry, but you need to learn a bit more about basic
electronics. ;)
If you have two transmitters wired in parallell, both
outputting +12V,
the result will be +12V, not +24V.
The receiver is never at risk in this setup. However, the
transmitters
might be. If one tries to output +12V and the other tries to
output
-12V, they will work against each other, and that could
possibly
overload the drivers, burning them up. Most circuits
nowadays have
current protection built in, so that they will not drive the
output with
more than a few hundred milliamps at most, which is low
enough to not
start frying things.
In order to get +24V, you should connect the +12V output
from the first
transmitter to ground on the second, and then measure
between ground of
the first one, and +12V of the second one. Then you'll see
+24V.
Same a battery.
However, for data communication, having several transmitters
will most
of the time simply not work. The reason is the thing
mentioned above.
RS-232 uses -+ voltage for 0 and 1. And they are both
actively driven by
most circuits. Which means they must both be transmitting
the same data
at the same time for the signal to get to the receiver
correctly. If one
wants to transmit a 0, and the other a 1, one of the signals
will
definitely be lost. And we could dive deeper into this, but
suffice to
say that it's not neccesarily transmitter A who always wins
over
transmitter B either (but if that was the case, then it
would be
equivalent to just having transmitter A connected).
So data will be garbled based just on the voltage levels.
(Oh, and RS-232 is specified as something like 5-15 volts,
both + and -,
so we have a fairly large range to play with, and even
though 0V isn't
actually legal, it is usually accepted as the same as a
negative level).
Then you also have the timing issues, as mentioned, the
"normally"
asynchronous characteristic of the signal.
If you really want to play with several transmitters to one
receiver, it
can be done under special circumstances. If you have
transmitters that
only drive a negative voltage, and leaves the output in a
floating state
instead of feeding the positive, you can then wire the
positive with a
pullup resistor. That will allow any of the transmitters to
pull down
the signal, and at idle, the signal stays positive. If you
then
(somehow) make sure that only one transmitter at a time is
sending, then
you will have a working setup.
Johnny
James K. Lowden wrote:
> Tomas D wrote:
>
>>what
>>would happen if both systems tried to send data over
serial connection
>>at the same time?
>
>
> At best the receiving device would get garbage. At
worst you'd damage one
> or more devices.
>
> RS-232 works by raising and lowering voltages on the
signal lines. The
> succession of voltage/no voltage detected on the
receiving end is
> interpreted as a series of ones and zeros.
>
> For it to work, the timing in critical. (You're
probably accustomed to
> using RS-232 in async mode, but it can be used in
synchronous mode to,
> where some of those 25 pins are dedicated to carry a
time signal.) Two
> machines transmitting over the same wire will not be
able to coordinate
> their timing. Consequently the receiver will see not a
neat succession of
> on/off, but some "syncopated" sequence it
won't be able to interpret.
>
> As for the worst case, remember that the RS-232 spec
calls for voltages of
> IIRC +/- 12 volts. If two machines apply 12 volts
simultaneously, there'd
> be 24 on the line. Let's only hope all the attached
hardware is designed
> to tolerate 2x the specificed voltage, because
otherwise it's toast.
>
> Of course, this is just a specific instance of the
general case: for two
> processes to share a device, either they have to
cooperate or they need an
> arbitrator.
>
> HTH.
>
> --jkl
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt update.uu.se || Reading murder books
pdp is alive! || tryin' to stay
hip" - B. Idol
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