Simple data bus over DC Power Wires

There just may be:

You want cheap:

L1 DC source ----))))))))----------------------- To others or load ! ! \ R1 --- C1 / --- \ ! ! R4 / R2 ! -/\/\/--- \ !--------------! ! ! ! / R3 ! Micro TXD!---!!- ! /+!------/\/\/-- Vcc/2 ! ! C2 ! / ! ! RXD!------------ ! U1 !--------------! \ ! \-!---- Vcc/2

C2, R1 and L1 turn the RS232 output of the micro into positive and negitive going glitches.

L1 blocks the AC from going into the supply or load. It is in parallel with all the others. The total inductance interacts with C2 to define the width of the glitches.

R1 damps the L1 C2 combination to make the responce not overshoot.

C1 picks the glitches off the power line and applies them to U1. U1 is configured to have enough positive feedback that it doesn't chatter.

Now all you need is software.

This method has been done over 1000 foot cables.

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kensmith@rahul.net   forging knowledge

You could always go analog. Each switch has a series LC network, and the control circuit just imposes a small AC frequency over the network, and looks at the impedance. The plusses of this are of course you only need one real circuit, the others can practically be built onto a chocolate block.

I looked there, but couldn't see anything related to multi-channel control over the DC supply wires.

I have a pal who (in later life) is resurrecting his interest in model railways. Apparently there is now some reasonably standardised method of multi-device control via the DC supply through the rails. Trains, points and signals can all be controlled via one DC supply. It is supposed to be popular in the US model engineering market, but he hasn't been able to find out any technical details yet.

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Tony Williams.

I thought someone would mention this eventually. It's called Digital Command Control (DCC) and is maintained by the National Model Railroad Association. The NMRA standards are at

There are a lot of manufacturers world-wide that make the various mobile and staionary decoders, booster/command stations, hand-held throttles, computer interfaces, etc. Note that the standard only applies to the booster to decoder side, while the booster / throttle / computer network tends to be proprietary to each manufacturer. Most of the modelers I know use Digitrax

Mike

Thanks for the information. I've bounced your reply to my pal.

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Tony Williams.

what would be the disadvantage of removing U1 and all connected parts on receiving side and connecting C1 directly to RXD grounded through input impedance resistor ? Now that's inexpensive :)

As for the protection of RXD from spikes, I would consider the built-in clamp diodes (some microcontrollers have them) together with R2.

Roman

I don't think that DCC is suitable because it looks they reverse the polarity on the rails or some crazy thing. The Siemens ASI seems to be it. It uses a simple unshielded two wire data+power cable and the network can be up to 64 slaves each of which can be multiple I/O. Noise immunity is achieved by superimposing a narrowband sinewave that is modulated by something called APM= Alternate Pulse Modulation- of just exactly what-phase, amplitude, or something else-I don't know, but the slave receivers require crystal timebases. The ASI has been around for

10 years now so that it is well-developed and supported- you can tie it into almost any other industrial control bus, or PCI, or VME, or whatever- and there are plenty of chipsets available for building a product from scratch too. It has quite a bit of protocol overhead so that custom development would not make sense unless you're building a compatible product for market- an in-house actuator control bus should use readily available off-the-shelf modules from any one of dozens of manufacturers.

Yes, I was quite surprised to see that. I had imagined some sort of dc rail, modulated with an mf two-tone (or something). Using a full bipolar switching scheme seems a hard way to go about it.

However it seems that the model railway market is all going DCC, so that's the way my pal will have to go as well.

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Tony Williams.

Try

Paul Burke

Interesting Ken,

When you say, this has been used up to 1000 ft, you actually implemented it?

If so, how reliable was it and what kind of data rates did you get?

Thanks for the feedback,

Paul

Interesting Ken,

When you say, this has been used up to 1000 ft, you actually implemented it?

If so, how reliable was it and what kind of data rates did you get?

Thanks for the feedback,

Paul

Any idea what it would cost per node to implemented ASI, and how complex is it?

Now sure how much the ASI chips cost. Siemens makes them I think.

Paul

If you do that, the micro gets something like this on its input:

!- ! \ ! \ ! \

---- ------- ------------------------ ! / ! / ! / !-

U1 turns that into:

----------- ! !

-------! !----------------------------

Which is very like the other unit's TXD signal.

If the micro has an ADC you could perhaps do without the U1.

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kensmith@rahul.net   forging knowledge

Yes and sold it to someone. The circuit was a little different because we used an LT1081 to make the signal from the micro bigger and stronger and attempted to cancel the self signal (the micro hearing its self).

It was about 100% for one side only transmitting. The self signal cancelation was a bad idea. It is better, if you can, just to only have one side talk.

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kensmith@rahul.net   forging knowledge

How about the following circuit

L1 DC source ----))))))))----------o-------------- To others or load ! --- C1 --- ! / \ R2 !--------------! / ! Micro TXD!---! ! ! ! ! ! R3 ! RXD!---o------------o----/\/\/----! GND !--------------!

The high pass filter R2,R3,C should work with reasonable (R2+R3)*C time constant, i.e. should carry over everything from first harmonic, which is BR/N, where N = databits+stopbits+startbits.

Let's consider 2 extreme cases, one with all bits ones and one with all bits zeroes for 8N1.

In case of all zeroes (8N1):

!-! !-! !-! ! ! ! ! ! !

-!-! !-!-!-!-!-!-!-!-!-! !-!-!-!-!-!-!-!-!-! !- x S x S x S

S = Start Bit x = Stop Bit

DC component of 1/10th signallig voltage will be stripped

1st harmonic of BR/10 and everything above will be carried over. Microcontroller will receive correct data. The recessive state will be slightly negative.

In case of all ones:

!-!-!-!-!-!-!-!-!-! !-!-!-!-!-!-!-!-!-! !-!- ! ! ! ! !

-!-! !-! !-! x S x S x S

DC component of 9/10th signallig voltage will be stripped (too bad)

1st harmonic of BR/10 and everything above will be carried over. The recessive state will be negative, the dominant state will be slightly over ground. Microcontroller would not receive correct data unless DC component is restored.

As for restoring DC component, one example I can think of is if R2

Hello, You could also look into a company called Yamar (

) who makes chips that communicate over the DC power lines using the LIN bus. Soon there will be modules that can implement 250K CAN bus. They are not very expensive. You can follow the electric car charging standard. Once this is final then I imagine more suppliers will be making parts. If you want data and power then look into a company called PowerByProxy out of NZ. This is more expensive.

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