furthest distance between two 8031

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Hi,

What is the longest distance that can be achieved just using the TX, RX and
GND to communicate between two 8031s at 2400 & 9600 b/s where the interface
is:

1. direct connection (TTL).
2. through a pair of MAX232.
3. using 26LS31/26LS32 transceivers.

Should I use twisted pair wires or straight parallel wires to improve the
transmission?


Allen





------
The next war will determine NOT who is right  BUT what is left.




Re: furthest distance between two 8031

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and
interface

If you're going to a system with different power supplies or out of the same
box, do not attempt without drivers of some kind.

Your speeds are so slow that you can get reasonable distance with RS-232
drivers/receivers (the maximum in standard is 50 feet / 15 meters).

Probably the highest speed with simple drivers is reached with RS-322/RS-485
drivers/receivers. Even here the ground differences must be moderate (not
much over a volt). Here with suitable cable and ground noise in control, the
max distance should be at least hundreds of meters.

For longer distances, consider using a pair of inexpensive modems.

HTH

Tauno Voipio
tauno voipio @ iki fi




Re: furthest distance between two 8031
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The RS485/422 is specified as 1 mile or so at whatever baudrate.

Rnee
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net


Re: furthest distance between two 8031

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the
same
RS-322/RS-485
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(not
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the

Yep - subject to the common-mode limitation, which often is not possible in
practice, even with shorter cables.

Tauno Voipio
tauno voipio @ iki fi



Re: furthest distance between two 8031
: >
: > The RS485/422 is specified as 1 mile or so at whatever baudrate.
: >
:
: Yep - subject to the common-mode limitation, which often is not possible
in
: practice, even with shorter cables.
:

    You can get optoisolated inputs on those, which will remove that voltage
difference (on ground).   Done 1/4 mile no problem that way.  Well
realistically 1000' (the whole spool) , not a 1/4 mile, both units getting
power from different panels with different grounds.



Re: furthest distance between two 8031
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True. Well, the GND should be passed along with the signal.

Rene


Re: furthest distance between two 8031
On Wed, 27 Aug 2003 19:09:16 GMT, Rene Tschaggelar

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And what do you expect to gain by doing that ?

The common mode voltage difference is still there in most cases and
now you have the often dirty ground loop current flowing all around in
quite thin wires and PCB strips.

The most common reason for ground potential differences at the end of
the serial link are the different electric distribution systems in the
ends of the link. In most systems, the signal ground is tied to the
mains protective ground, which is tied to the mains neutral in the
mains socket or in the local or main distribution panel.

In single phase systems and badly balanced two phase (America) or
three phase (the rest of the world) distribution systems, a
considerable current can flow in the neutral wire, causing some
voltage drop in the neutral conductor.  This potential is propagated
through the protective ground to the signal ground of the equipment.

Since the neutral current and neutral wire resistance is usually
different in different places, there is going to be a potential
difference between the ends of the serial cable.

Adding a thin signal ground wire does not help a lot to remove the
potential difference between the equipment, although some current (a
few amps at most) will flow in the signal ground wire.

To be really effective in removing the ground potential difference,
the signal ground wire must have a cross section area many times the
neutral wire cross section and it may have to carry up to tens of
amperes of ground current in the worst case.

Thus, for long distance serial connections, especially between
different buildings (in which thunderstorm related ground potential
differences can be quite large), the best way is to totally isolate at
least one end of the link from the local ground. In this way, there
can be no ground loop currents or common mode voltages at the
transceivers.

However, if the ground potential difference is known to be at all
times within the transceiver common mode range (e.g. in the same house
and the electric distribution branch), a simple twisted pair should be
enough with termination resistors at the receivers. The transceiver
grounds can be connected to the respective local protective ground. If
a shielded cable is used, the shield can be connected to ground at one
end only.

Paul
      

Re: furthest distance between two 8031
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This :

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isolate one end. Then it is useful to have the GND belonging
to the signal. Have a floating supply for the dangling end.
Some even spend another wire and pass the Vcc and the GND along.
Thus no floating supply is needed for the isolation.

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Yes.

Rene


Re: furthest distance between two 8031
On Thu, 28 Aug 2003 18:12:43 GMT, Rene Tschaggelar

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In this situation the only reason I can think of for a separate ground
wire in a typical RS-422/485 point to point connection is to provide
bias current for the receiver input transistors.

However, with a typical fail safe termination system, with the actual
termination resistor between Rx+ and Rx- and pull up resistors to the
local receiver Vcc and Gnd, the receiver transistors will receive the
correct bias current without a separate Gnd wire even when the remote
end is floating.
      
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Yes, this is a useful configuration.

Paul


Re: furthest distance between two 8031

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At speeds up to 9600 (or even 38400 bit/s) optoisolated 20 mA current
loop would be the simplest solution. Use the slowest practical speed
and some low pass filtering matched to the line speed may also help
reduce transmission errors.

Optoisolated RS-422 is also a good solution. If different variations
of the transceiver chips, one rated at say up to 250 kbit/s and the
other up to 2 Mbit/s, use the slower one if your application permits
it, since it will better tolerate very short interference peaks. Of
course external low pass filtering tailored for the transmission speed
can be used at both the transmitter end (to slow down transitions and
thus reduce crosstalk) and at the receiver end (to reduce sensitivity
to interference peaks).

One should remember that when using metallic conductors transferring
signals between a very noisy environment and to a less noisy
environment, any noise picked up by the conductors (or even the
metallic cable shield) will propagate through the wall to the less
noisy side and radiate into it, unless some feed through filtering is
used at the wall.

If the transfer speed needs to be large, there exist huge potential
differences between the ends or there might be problems due to
interference conducted through metallic conductors from one area to an
other, using optical fibers is the simplest solution.

Paul
      

Re: furthest distance between two 8031
Is it possible to use voltage reference to transfer signal beside 1010?
etc
transfer 24 bits condition from one end to the other using voltage reference
from 0 - 5V or 0 - 12V or even 0 - 24V by mean of DAC to ADC conversion.

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25m



Re: furthest distance between two 8031

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RX
the
RS-232
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control,
in

I'd like to read more on this "common-mode litmitation" subject.  Any
recommendation for a good link?

Allen

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Re: furthest distance between two 8031

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the
same

I need to communicate a distance of about 75 to 100 metres in the same
building.

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RS-322/RS-485
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the

My 8031 board came with a pair of 75176 with the signals -422XMIT , GND and
+422RCV.  Should I use a twisted pair for the TX and RX signal or just use
audio screened wires with the ground terminated on one end?

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How do I use 2 modems without going through an telephone exchange.  The 2
wires coming out of the modem would be floating.

Allen

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