my friend got the (theoretical) idea to connect CAN controller to AM keyed power line transceiver. CAN already handles arbitration, retries and CRC. Of course we talk low speed CAN.
Or perhaps LIN over power lines. Not as complete as CAN, but dropped packet would not be as expensive as dropped CAN packet due to lower overhead.
Anybody thought of this, or developed something similar ?
CAN requires that a received bit have more or less the same magnitude as a transmitted bit, so the bit-by-bit arbitration will work. So CAN isn't going to work over any medium that looks like radio. Since power lines generally attenuate the signal severely, and the AC power line is an example, rather than an exception, of this.
So CAN is out. I don't know if LIN would work, because I'm totally unfamiliar with it. You need a protocol that does combined backward-error-correction/collision detection using at least an ACK and a timeout, perhaps with a NACK to speed things up if you're clever with your CRCs.
Tim, I was talking about the controller, PHY would be specific for PL.
Btw. I am not married to this idea, have no project, it simply popped up in conversation and my response was similar to yours but when I thought more about it, more I liked it.
Let me quote from
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Physical media The basis for transmitting CAN messages and for competing for bus access is the ability to represent a dominant and a recessive bit value. This is possible for electrical and optical media so far. Also powerline and wireless transmission is possible.
ASK is a great candidate for such competing.
Now correct me if I am wrong, because the following is currently outside my expertise and I just blurt this out:
PL are parallel wires similar to twisted pair, so they have similar EM characteristics but lower resistance (due to gauge) and lower capacitance (due to thicker dielectric) and low impedance termination (appliances).
The impedance of connected appliances can be very low (say 2ohm for
5-7kW from 110V). In extreme case: Zout+Zwire = 1000 ohm Zload = 1 ohm the attenuation would be 60dB.
But ... 60dB is not a problem for SW receiver, is it ?
Please note that for AM the needed bandwidth is two times the highest modulating frequency.
Consider also that quickly changing signals travel in the insulator between the transmission line conductors, in extreme cases one of the conductors may be left out as in waveguides.
The normal house wiring resembles the anetnna constructions of Messrs Marconi and Popov. How on earth can the poor RF energy know that in this case the signal should be kept within the wiring and not radiated out?
There are good physical reasons why fast data will never travel cleanly in current house AC wiring - forget it.
The biggest problem is noise. I asked several people to give me an idea on the amount of noise on powerlines, but nobody really knew. Googling did not help. It was time for an experiment:
The problem with CAN is that you need a means to implement the dominant and recessive state and a means to monitor your own transmission to see if your recessive state has been changed to a dominant state and I do not see how this could happen on the power line.
Of course it is possible to connect two separate CAN networks together with any communication channel, including radio or power lines. But they are separate networks after all (use fixed width font):
A X ! ! B---+--P- - - - - - - - - - - Q--+---Y ! ! C Z
One CAN network consists of nodes A, B, C and the gateway P and the other CAN network of nodes X, Y, Z and the gateway Q. The connection between P and Q can be any (bidirectional) communication channel.
A message generated by A is received by B, C and P. B or C might react upon it, anyhow P will send the message via the communication channel to Q, which puts the whole message into a queue. Q now performs a new arbitration into CAN network XYZQ and may have to wait a while, before the message from A can be delivered to X, Y and Z. If there is a severe congestion at this network, Q might even rearrange the queue according to the CAN-ID message priority.
The situation is similar for messages originated at say X.
It should be noted that the frame delay can be several frame transfer times (especially with a half-duplex radio link or internet connection to the other side of the world) and the gateway receiving a frame from the communication link must be able to buffer it and perform normal CAN arbitration in order to enter the new network.
The situation gets complicated, if the capacity between P and Q is less than the individual networks. The gateway may have to filter out messages based on the message-ID. This must either be manually configured or the gateway may determine, which traffic is local to either network (such as detecting local RTR traffic or knowing something about the high level protocol, e.g. in A is CanOpen master and can directly communicate with SDO requests with local nodes B and C, while only SDO requests to X, Y or Z will go through the gateway.
Of course the network could be simplified to
B----P- - - - - - - - - - - Q----Y
but why use CAN networks between B-P and Q-Y, when you still have to do a media conversion at P and Q ?
There are SEVERAL implementations . Beer-Can was but one. This was done by someone at Siemens (now Infinieon ) in the UK to work with their 16* MCUs However I have come across *several* other companies with CAN over (AC) power lines over the years. However as CAN is not one of my areas of expertise (or interest) I have not kept the notes.
If you look on the main CAN web sites you should find some information on it. As CAN is mainly used in automotive systems the CAN over AC power is not common but it certainly has been done by more than once and AFAIK is in industrial use both sides of the Atlantic.
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You could use a Power Harmonic Analyser. The one we use at my univeristy are from Tektronics, they have a basic GUI (graphical user interface) with a lot of statistics and of course the FFT (fast fourrier transformation) witch will give you a pretty good idea of the harmonics and their respective power in db (decibel). We've been using it with 3 phase motor control using thyristors. I've never tried to check the power line myself but I can tell you this : BE EXTRA CAREFULL.
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