RS232 distance

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SilentScreamer

Senior Member
This is a really quick question but I can't seem to find an answer. How far can I realistically send serial data from a PICAXE without the need for additional hardware? I know it will depend on many things but I just need a ball park figure.
 
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eclectic

Moderator
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Tim036

Member
OK, at a slow baud rate you might get a mile. If it starts at +15v for a logic 0 and -15 volts for a logic 1 you will get a lot farther than + and - 5 volts !

Cable matters as well.

Using good Driver chips 1488 and 1489 (from memory) will help.

If you want 9600 baud or above be more conservative.

In the early days it was 110 baud as the norm !

Its a bit of a 'battleship' data transfer protocol. Plagued by having to match both ends exactly on its many setting ! but tough as old boots as it can be without error correction.

:D

Tim
 
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BCJKiwi

Senior Member
Biggest issue with RS232 over long distances is that these are typically on separate power supplies so all manner of potential (power) differences can be present leading to failures of the RS232 modules at either (or both) ends.
 
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SilentScreamer

Senior Member
Both units run from the same power supply so that won't be an issue. I'd heard that RS232 could handle hundreds of meters with ease but I didn't know if the 5V/0V PICAXE serial could. I only need two PICAXE's communicating over 100m at 9600 baud so it should be easy(ish). :D
 
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Tim036

Member
SilentScreamer said:
Both units run from the same power supply so that won't be an issue. I'd heard that RS232 could handle hundreds of meters with ease but I didn't know if the 5V/0V PICAXE serial could. I only need two PICAXE's communicating over 100m at 9600 baud so it should be easy(ish). :D
Click to expand...

1448 / 1489 are cheap and easy, so not worth risking the challenge of 0v and +5v will bring IMHO.

:)

Tim
 
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manie

Senior Member
Silent: Make it easy on yourself, get the extra hardware, like good 433MHz and Manuka's "Cotanga" Yagi, I did 1.5Km no problem...
 
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manuka

Senior Member
Thanks to 433 MHz wireless data, its getting some years back (~2003) now since we did this, but diverse PICAXE serial party tricks abound at slower (< 1200 bps) data rates. Why we even used damp strings, chains of kids holding hands, wire clothes lines & fence wire/ground return. I don't recall a range limit ever being reached, but hundreds of metres was typical. Stan
 
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Dippy

Moderator
In that link, supplied by ec, Drac said he could get much further than that with twisted pair.

To me, that suggests its worth experimenting - maybe with different types of cable and different bauds.
So, go and buy some cable and try it out :)

Alternatively, you could get some kids to hold hands 20m underwater.
 
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Tim036

Member
Phono cable/leads might be a good bet being driven from 150 ohms into 150 ohms.

Should be more robust than 600 ohms and closer to the natural impedance of the cable.

:)

Tim
 
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Dippy

Moderator
There you go then SS.

Nip down Maplins.
Get:-
20m phono cable
20m bell-wire
20m coax
20m of twisted pair or phone cable.

Careful you don't slip on the kerb or else you'll end up with a twisted pair :)

You'll probably find that they'll all work under ideal conditions.
 
hippy

hippy

Ex-Staff (retired)
There's perhaps no better method than trying it and seeing what happens, or setting up a simulation which models the cable you are planning to use, though reality may differ to simulation. Both resistance and capacitance will affect the signal.

For mission critical systems it's usually best practice to over-design so that the setup will work and then reduce from there. A matter of 'will any cost savings be outweighed by increased risk of failure'.
 
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BeanieBots

Moderator
You should be able to get pF/m and R/m values for the cable so simulating (either with a calculator or breadboard) should be reasonably close to reality.

If you leave the wire coiled on the drum and the test works, you can be sure it will work when layed out straight. (C/m & L/m are much higher when coiled on the drum).
 
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manuka

Senior Member
??? Where did this "under water" aspect come from???? Fibre optics in such a situation makes more sense.
 
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Dippy

Moderator
"??? Where did this "under water" aspect come from????"
- well, at a guess, he wants to use it underwater maybe?

"Fibre optics in such a situation makes more sense."
- I just knew we'd get onto fibre optics...:)
... but, yes, it would be nice. However, I can only assume he wants to keep it dead simple with no added chips etc.

Nesbit, here we come.
 
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BeanieBots

Moderator
Good point, missed the water side of it.
The ONLY cable solution for underwater comms is co-ax.
Twisted pair etc don't behave like twisted pair when surrounded in close proximity by conductive dialectrics like water.

Fibre optics would be a good choice but as pointed out, has the complexity of interfacing to consider.

The distance is quite short so I'd go for a co-ax solution. Cheap cable, water resistant and easy to test on land with comparable results to those found under water. The choice of cable would also depend on the physical aspects. Many of the 75ohm TV signal type cables are not suitable for small radius bends and/or constant flexing.
 
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