RS 485 Communication

premkumar9 · Mar 25, 2011

Hi
For a project, I need to collect data from locations up to 500 to 800 meters away from my main equipment. It will be a two way communication. I plan to use RS485 communication - with RS485 tranciever ICs ( I use microconrtollers at both sides).

1) Is this a suitable method for my application?
2) Any extra precautions required to avoid noise problems?
3) can I use baud rate of 9600 for such distances?
4) Is there any better method?

Looking forward to your valuable suggestions.

JimB · Mar 25, 2011

premkumar9 said:
Hi
For a project, I need to collect data from locations up to 500 to 800 meters away from my main equipment. It will be a two way communication. I plan to use RS485 communication - with RS485 tranciever ICs ( I use microconrtollers at both sides).

1) Is this a suitable method for my application? JimB: Yes

2) Any extra precautions required to avoid noise problems? JimB: Use "twin twisted" cable, screening may be an advantage if the envirinment is electrically noisy.
Make sure that the cable is correctly terminated (150ohm ?)

3) can I use baud rate of 9600 for such distances? JimB: Yes, no problem. You may be able to go faster, give it a try.

4) Is there any better method? JimB: I dont know. RS485 is well understood, simple and works without any fuss.

Looking forward to your valuable suggestions.

Extra comment to make up a minimum character count.

JimB

premkumar9 · Mar 25, 2011

Thank you.
Regarding termination resistance - The IC specs I used give "Receiver Input Impedance . . . 12 kW Min". That means I have to connect an extra resistance equal to the characteristic impedence of the transmission cable. Is it right?
Is it required at both ends?
The value you mentioned (150 ohm) is standard for all twisted pair cable used for this purpose OR will vary for different twisted pair cables?

Extra comment to make up a minimum character count.

What does it mean?

JimB · Mar 25, 2011

premkumar9 said:
Thank you.
Regarding termination resistance - The IC specs I used give "Receiver Input Impedance . . . 12 kW Min".

The receiver input impedance is high so that many devices may be multi-dropped off the comms line without creating impedance discontinuities.

premkumar9 said:
That means I have to connect an extra resistance equal to the characteristic impedence of the transmission cable. Is it right?
Is it required at both ends?

Yes and Yes.

premkumar9 said:
The value you mentioned (150 ohm) is standard for all twisted pair cable used for this purpose OR will vary for different twisted pair cables?

The characteristic impedance of a twisted pair cable depends on the wire diameter, the spacing between the wires and the dielectric constant of the insulation.
But as a general rule the impedance will be in the range 75 to 150 ohm.
A quick look at the MAX485 datasheet shows examples with 120ohm terminating resistors at both ends of the line.

premkumar9 said:
Extra comment to make up a minimum character count.

Click to expand...

What does it mean?

Because I had made a reply by by effectively editing a copy of your post, the forum software thought that I had not written anything and so it objected when I clicked on Submit Reply.
So the extra comment was added to make up the minumim character count. (Did I just say that?)

JimB

premkumar9 · Mar 25, 2011

Thank you for your clear answers on each point.
Have you used RS485 communication for distances above 500 meters?

JimB · Mar 26, 2011

premkumar9 said:
Have you used RS485 communication for distances above 500 meters?

My personal experience is probably limited to about 100metres.

JimB

premkumar9 · Mar 26, 2011

I am only doing the lab testing right now. I may have to use it later for few hundred meters length in an environment where there would be lot of electrical noise. I think, in case there is any issues due to noise pick ups in spite of precautions like using twisted pair cable with shielding etc, I can use some repeaters . What is your opinion?

JimB · Mar 26, 2011

What is the environment, what are the sources of electrical noise?

A repeater sounds like overkill and I am not sure how you would implement it on a two wire bi-directional line.

JimB

be80be · Mar 26, 2011

Maximum Distance 1200 metres (4000 feet)

With RS485 on the contrary there is no such thing as a common zero as a signal reference. Several volts difference in the ground level of the RS485 transmitter and receiver does not cause any problems. The RS485 signals are floating and each signal is transmitted over a Sig+ line and a Sig- line. The RS485 receiver compares the voltage difference between both lines, instead of the absolute voltage level on a signal line. This works well and prevents the existence of ground loops, a common source of communication problems. The best results are achieved if the Sig+ and Sig- lines are twisted. The image below explains why.

**broken link removed**

In the picture above, noise is generated by magnetic fields from the environment. The picture shows the magnetic field lines and the noise current in the RS485 data lines that is the result of that magnetic field. In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer. When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable.

And the rest of the story https://www.lammertbies.nl/comm/info/RS-485.html

premkumar9 · Mar 27, 2011

And the rest of the story RS485, specifications and in depth tutorial

Thank you very much

premkumar9 · Mar 27, 2011

JimB said:
What is the environment, what are the sources of electrical noise?

Water pumps, Railway engines, Electric welding etc.

JimB · Mar 27, 2011

premkumar9 said:
Water pumps,

Just keep the comms cable away from the power cables as much as possible.

premkumar9 said:
Railway engines,

Again, keep the comms cable away from the power cables as much as possible.

No EMC problems with this railway engine!
The A1 Steam Locomotive Trust

premkumar9 said:
Electric welding etc.

Depending on the environment, electric welding operations can move around, just do the best yoy can to keep the comms cables away from the welding areas.

JimB

premkumar9 · Mar 27, 2011

Thank you. I will keep these points in mind while installation.

PappuNeelam · Mar 27, 2011

Hi premKumar,

Its good to use RS485 network, because u can easily get a twisted pair from CAT-5E cablesused in networking]. The minimum requirement of RS485 network is the characteristics impedence of 100OHM. & if u r using only two processor in the network, no need of Termination register (120OHMS) to avoid data reflections. Also, no need of biasing with 620OHMS registers as u said that there would be no passive nodes.
Only use two RS-232 to RS485 Converter[converting RS232 logic to RS485 logic], if using PC DB9 port. Otherwise, use MAX485 chip to convert the TTL/CMOS serial logic to RS485 logic and transmit the serial data through one pair twisted cable.
Pappu

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