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Author Topic: comms using a current loop  (Read 323 times)
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I've played with RS485 and TTL comms before.
I've come across this interesting page, and I realise at the base of it all, It'll be digitalWrite type bit banging to make this type of comms happen.

What I'm unclear on is how I interface to the current loop as the master?
http://www.vwf.com.sg/vwf/pumpcomms.html

I think opto isolaters will be a must, but I'm not sure what goes between the opto isolater and the + and - cabling from the current loop..?
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The point of this scheme,  is that it is almost immune to electrical interference.   What you need to do,   is build a "constant current source"  which generates 45 mA,    and then keep turning it on and off according to the binary code you are trying to send.       An easy way to make a constant current source is I think using a LM317 regulator    and setting it up with the right external resistors and feedback  to make it into a constant current device.

Google "LM317 constant current",   should give you some places to start looking.    You then need to devise a way to turn it on and off at the appropriate timing,   should not be too hard.
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current loops are relatively immune to capacitive interference, which is high-impedance,
because its a low-impedance technique, but that won't protect it from inductive interference
per-se, its being a high-voltage signalling scheme that does that.  So current loops running
at 20V or so are going to withstand loads of full-on EMI, but at a cost of lots of power
wasted.

A differential signalling scheme is probably a better performer, since cancelling out
interference to a first-order allows lower voltages and currents for the same robustness.

(PS the name is a bit odd really, all circuits are current loops!)

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Yeh, I'll be the 'poller', the other end likely has the resistors and such already in  place.
To be the client, it sounds like all I need to do is add a transformer, I'm not yet sure if it's AC or DC, and what potential.

To find that out, I suppose I need to know the resistance on the other end...
Then, to talk to it, the opto isolator should be all that is needed?
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You might want to look up some MIDI circuits, which also use a current loop...
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