Data transmission by wire for 10 km

[predeina.alexandra19]

Hello dear forum users!
I will describe my task and my current thoughts about its solution.

The challenge is to transmit signals in both directions over a distance of 3 to 10 km and over ordinary wires. The wires are not a differential bus, just two parallel wires lying side by side. (like a old telephone wire).
The speed is really not very important, some 200-1000 bytes/s will be pretty enough.

After studying the topic and making several experiments, I came to the conclusion that the best option would be to use two microcontrollers, each of which will have 2 SPI modules and 1 UART module. Receiver-Transmitter "Manchester-2" will be implemented on 2 SPI modules and CLC modules, and UART is required to receive and transmit information from an external device.

Up to this point, I have considered HOLT microcircuits, including an encoder / decoder (HI-15530) and a transceiver with transformers. As I understand it, the microcircuit encodes a stream of bytes from the microcontroller, converts this stream into a differential signal and then transmits the signal through the transceiver to the line. I purchased several of these microcircuits and I was able to launch and receive data. But in this example I don't like two points, the first is the limitation of the size of one package to 2 bytes (16 bits) and the second is the high price. In this case, after the transmission of two bytes, a "tail" is formed, which greatly spoils the signal and does not allow the transmission of the next packets faster than the signal is established. (see the picture in the appendix).

At the moment, I purchased a PIC16F15323 microcontroller with one SPI module and 4 CLC cells for testing. I set up SPI, set up CLC and received at the output an SPI signal encoded by "Manchester-2", which I could quite successfully read with a logic analyzer (see the second picture). I suppose, with the one NPN bipolar transistor (or an inverter on the CLC), I can "unwrap" the signal, get a differential signal "Manchester-2", and then feed it to some transceiver, like CAN-BUS or RS485, and get something same but not limited to 2 bytes?

Please tell me if my train of thought is correct and is such a task feasible at all? Or maybe there is some more correct and easier approach?

Thank you in advance for your answer,
Yours faithfully,
Alexandra.

 

[betwixt]

I'm lost here. Transmitting low speed data over wires doesn't need Manchester encoding. The wire type is far from ideal but as long as the wires remain parallel all the way I would try RS485. I'm not sure why you think it is limited to 2 bytes. There is no theoretical limit to data length with RS485 and it can be bi-directional (half duplex). You won't get high speeds over that length of wires with any protocol or signalling system but that is a physical limitation, lower speeds will be recoverable over longer distances.

Brian.

 

[predeina.alexandra19]

Good day Brian, thanks for the answer!

When I talked about the limitation of 2 bytes, I spoke specifically about the 15530 IC, this is its internal limitation.

Do you really think that RS485 can transmit information over such a long distance without problems? I read that the maximum distance for such a protocol is 1200 meters.

Alexandra.

 

[KlausST]

Hi,

what about good old analog modem?
Frequency modulated.
But I have no experience in that long distance wired communcation.

Klaus

 

[danadakk]

You have high common mode for sure. Sounds like a natural for a fiber solution. S/N, Reliability,
CM almost irrelevant, cost, all pretty good.

If you look at POTs from the pleastocene era distance between repeaters was 3,000 to 6,000 feet
for voice bandwidth roughly same as your low datarate of ~ 2K b/s (bits/sec).

DSL modem to 18,000 feet - https://whatis.techtarget.com/refer...l, the maximum range,gauge of the copper wire.


Regards, Dana.

 

[betwixt]

As you can see there are several solutions. The (very) old telegraph system could manage 50Km using low speed current reversal using something akin to Morse code. The theoretical practical limit to the distance you can cover is caused by resistance of the cable and hence how much current it has to carry but in practice, things like inductance, mutual capacitance and impedance mismatches are what cause the real limit. The concept that applies in all cases is that DC will travel great distances but as the frequency increases, the achievable range reduces.

Parallel wires are not best, if you can use twisted wires you can get more consistent results and better noise immunity.

Brian.

 

[dick_freebird]

I wouldn't rule out RS-232 with its higher amplitude
and noise margin, given the low data rate.

 

[danadakk]

RS232 Specifications and standard - Lammert Bies

Practical discussion of the RS232 standard and its specifications and properties.

www.lammertbies.nl

RS232 might be a little challenged.


Regards, Dana.

 

[predeina.alexandra19]

Thank you all very much for your answers! There is a lot of food for thought.
I read a lot about the Manchester-2 code, it is actively used in underground wells for oil analysis, a 5 km cable is often used there. I thought that my task is similar in parameters.

In view of your answers, I have another question. Does it make sense to increase the RS-485 signal in amplitude? For example, raise it to 20V so that the attenuation on the cable does not affect the result so much?

Thank you!
Alexandra

 

[betwixt]

Increasing the amplitude would probably make matters worse. Move voltage means more current is needed both to overcome capacitance in the lines to each other and ground and in the terminating resistances.

There is a tradeoff, lower voltage is generally faster but more prone to noise.

As the speed requirement is low, you might consider using FSK, maybe two audio tones for 0 and 1. It would be far more prone to interference being introduced to the wires but also maybe easier to filter the received signal to eliminate them.

Brian.