Comms over nasty piece of thick copper wire

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Adrian Hudson

Member
Hi All,

I am thinking about building a central heating controller that draws data from my weather station to intelligently control my heating.

Please would anyone recommend the (bidirectional) communications mechanism between two X2 series PICAXE chips with the proviso that it must run over approximately 7 or 8 metres of existing three core 1 mm copper cable (not twisted) that is currently installed within the walls of my house. Running new twisted pair is hard work and I have only just decorated so I want to avoid that at all costs.

One of the units will hold the main logic and switch the heating (and communicate with my weather PC via an RFA020/1 pair) and the other unit (at the other end of this wire) will be a display and control unit.

Will 1-wire (which I have never used before) work okay over thick untwisted copper cable or would you recommend something else? I would prefer not radio as I have to get power to the remote control unit anyway.

Thanks for your help.

EDIT: I have just seen that a PICAXE can't be a 1-wire slave (or am I mistaken?) in which case 1-wire is out. The question still stands - what simple options are open to me for bidirectional comms (and power) over 3 conductors of 1mm copper wire about 8 metres?
 
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srnet

Senior Member
Well before trying anything complicated, you have to try standard serial.

Distance wise I would certainly expect it to work, 9600 baud, maybe faster.

How reliable it is would be entirely dependent on the particular location and its electrical environment.

If standard serial works, I would then design some form of error checked messages between the two units.

As you did not say, I assumed there are no other signals or power on the 3 core cable.
 
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Adrian Hudson

Member
Thanks for the link. Unfortunately, I can't find further info, other than that post. He mentioned User Projects / Communication but I can't find that.
 
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Adrian Hudson

Member
Thanks srnet. Yes, I was hoping to power the remote unit over the three core cable as well (5V) so bi-di serial is probably out. Would home grown half-duplex work do you think? i.e. send and receive (not at the same time) over a single pin.
 
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AllyCat

Senior Member
Hi Adrian,

Yes, certainly try serial ("RS232" with "TTL" levels) comms first. 8 metres isn't very far (I believe some have even managed I2C over that distance, but not recommended); if necessary you could drive from about 300 ohms, put a few nF across the lines to keep the signal "clean", run at 1200 baud and maybe configure the PICaxe inputs to Schmitt levels.

If you need to distribute power as well, then maybe use the CH pipework as a fourth conductor. Personally, I've been considering a PICaxe "three wire bus" (maybe even two wires + CH pipework) for a single-master, multi-slave network using a simple hardware interface (a transitor or two) to include the ability to re-program the chips! The "send" wire would also distribute power and the "receive" line employ tri-state/wired-OR. But I'm afraid it's well down my To Do list at the moment.

If "RS232" isn't suitable then, as you have full control over both ends of the system, you can potentially "bit bang" any protocol that you wish, including One Wire.

Cheers, Alan.
 
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Adrian Hudson

Member
@AllyCat: That's good news. I wasn't sure whether the serial output of these devices was meant for off-circuit board or not. I had thought about the pipes but unfortunately there are no pipes near the remote end. I will breadboard something up with a coil of wire and see what we can get going! Thanks.
 
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srnet

Senior Member
Lots of way this could be made to work, buts its a time and money play off.

One way that will work for sure, is to use the 3 wire cable for power and do the comms with a pair of bi-directional wireless modules, Rev Ed even sell them.
 
hippy

hippy

Ex-Staff (retired)
I can't see any systemic problems with running power plus bi-directional ( half duplex ) comms over 8 metes of three core 'mains cable'.
 
A

AllyCat

Senior Member
Adrian Hudson said:
Would home grown half-duplex work do you think? i.e. send and receive (not at the same time) over a single pin.
Click to expand...
Hi Adrian,

Yes, that's what I would try first. Put a 150 - 300 ohm resistor is series with the pin at each end to avoid accidents. Don't forget to switch the pin back to an input at the end of each message and perhaps (even more important) to an output before transmission.

Depending on the structure of your software and comms protocol, you might want to use the chip hardware (hserin/out) pin(s) at one or both ends (maybe even consider linking those pins at each end).

Cheers, Alan.
 
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DamonHD

Senior Member
If you want to go the wireless route I've been using RFM22/RFM23 modules with the PICAXE (18M2+) bit-banging the SPI protocol, as part of the OpenTRV heating control project at http://opentrv.org.uk/

I can point out the bits of code to you if you'd like.

Slight confession: I haven't completed the RX portion because I haven't needed to. I can detect that the right sort of packet (header) is being sent and that's all I need.

Rgds

Damon
 
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Adrian Hudson

Member
@hippy, @AllyCat: Thank you both. Will post back how I get on.

@DamonHD: Thanks, but I want to avoid too much wireless. The main controller is going to be wireless, I just wanted the slave to be wired as I think it will ultimately be easier (and a little cheaper!!)
 
J

John West

Senior Member
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srnet

Senior Member
hippy said:
I can't see any systemic problems with running power plus bi-directional ( half duplex ) comms over 8 metes of three core 'mains cable'.
Click to expand...
Neither can I, after all Ethernet over co-ax does the same sort of thing.

So systemically definitely possible.

As Rich Seifert said you can run Ethernet over barbed wire, but just how much do you want to spend on the electronics ?
 
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