Out here it runs the gamut. The exception being some Japanese cars. But even there the number of electric/electronic gizmos is inversely proportional to reliability. When we came from church last Sunday I wanted to roll down the rear right window in my wife's car (Toyota). It's electric. Click - nada. Zilch. Again. Arrgh. Well, at least this time it didn't quit in the rolled-down position. That'll be $200+ for the part again I guess (last time it was under warranty). Mine has cranks and should that ever break I am sure I can fix it with some scrap metal parts I have in the garage.
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They do a good job in that domain. But it was a hassle the first time when the window wouldn't roll up anymore. My wife could not go anywhere much because you don't want to park a car in a public lot with a window open.
I have been using CAN at 50-250 kbit/s up to several hundred meters in industrial environments, with dozens of nodes on the bus with or without a signal ground. I haven't seen big differences one way or the other. Of course the CAN transceiver on each node is isolated from the rest of the node.
The signal ground may help in some situations or it may worsen the situation, especially when a separate wire is running parallel to the twisted pair carrying the data (i.e. not cancelling any external field).
When isolated transceivers are used, they usually tolerate 0.5-2.5 kV common mode voltages. In order to cause data integrity problems, the common mode voltage would have to be translated to a differential voltage. When the transmitter goes to recessive state, the current stops flowing and no voltage drop is generated across the termination resistance.
Only if the stray capacitance from CAN-H to external ground is much different from the stray capacitance from CAN-L to external ground, a voltage difference could be generated across the termination resistance, when current from the wire with lower stray capacitance flows through the termination resistance to the other side with a larger stray capacitance.
Such alternators generate very strong slowly varying magnetic fields, but it will generate high frequency interference only if the slip rings are sparking (assuming synchronous generator).
If a separate signal ground is used, use a wire from the same quad pair in order to cancel out any voltage induced between the signal pair and the ground wire. However, when using a quad pair, the balance in the actual CAN-H and CAN-L lines may be worse than when using an ordinary twisted pair.
According to the J1939 standard, there are two configurations for cable: shielded twisted pair and un-shielded twisted pair. We are using a shielded configuration with two wires and a shield. The standard further states that the shield should only be grounded at a single point on the bus, at the point of the best ground. In our case, the shield is grounded at the engine ECM and the other end (at our box) is unterminated. So it seems this should be correct as per the standard.
The question is: is it reasonable to expect interference from the power alternator on the CANbus cable if it is run in very close proximity, or would this only happen when there is high frequency emissions from the sparking rings? If there is a reasonable expectation of a problem, what is the best way to avoid it and be consistent with the J1939 standard?
Please post below quoted text. Makes it easier for most of us.
Maybe you could assess the situation at the receiving end where you get errors. Hang a fast dual-channel scope from CAN-H to board ground and CAN-L to board ground. These are the signals the chip actually sees. But be careful, if the noise spikes are extremely bad you could blow the scope inputs. So you might have to initially do that with protection diodes against VCC and board ground. If you see noise with full swings against the rails that means trouble.
I found that in situations like this common mode filtering was required. This wasn't CAN bus but similar buses in the presence of large switched loads.
for a bus topology you need a high-impedance input that can switch to a low impedance output, the former is hard to do with a transformer in the input. 10base2 used expensive isolated powersupply modules to get power to the high impedance input
rg58: sounds like 10base2 (aka 'thin' ethernet), see above.
the 10base2 card I have in front of me reads 0 ohms from the coax shield to pins 9 and 10 of the DP8392CN chip, again and from the centre conductor to pin 14
Have you looked at the signal lines with a battery powered (floating) oscilloscope in differential (A-B) mode in order to determine, if this is a 50/60 Hz issue due to the magnetic field or some high frequency issue due to the slip rings ?
The twisted pair relies on cancelling induced interference during each turn, so make sure that a good quality twisted pair cable is used with a constant turns/m ratio. Also avoid sharp bends (created by an enthusiastic installer making a "pretty" wiring with sharp bends), which would destroy the symmetry.
If everything else fails, there are various "CAN extenders" using fiber optics, but these create extra propagation delay, thus reducing the maximum distance. However, at 250 kbit/s (J1939) this is usually not a problem.
When trying to measure differential voltages riding on a large (>10x) common mode voltage, it is important that the test equipment is truly floating (e.g. battery powered). Keep the A and B channel test leads close to each other and away from any grounded objects and put the test equipment away from any grounded objects (e.g. on a wooden chair) and do not touch the instrument or the test leads during the measurements.
Touching or even being close to one signal conductor only may increase the stray capacitance by more than 100 pF. This is a serious problem with high impedance systems, but with properly terminated RS-422/485/CAN systems with 50-120 ohm termination resistance, this is only an issue for common mode interference in the MHz range. The unbalanced stray capacitance will cause a current to flow in the load resistor and convert common mode interference to differential interference, which should be avoided.
For the bus one would have to define what constitutes a bus access. It can be done without DC coupling. But I guess once a protocol is in place it'll be too late. WRT power transfer we do isolated power transfers in medical electronics all the time. It isn't at all expensive. Except a one-time cost for agency certification but that's something med gear has to undergo anyhow.
The parts of the system that have patient contact typically need to be
100% isolated from everything that is mains connected. Including the bus.
I'll have to see whether I still have an old card somewhere and measure it.
I thought the bi-phase mode was supposed to be transformer coupled? OTOH I've never heard of anyone actually using that mode and not all controllers support it. I do seem to remember it neing psrt of the original chips though.
Robert
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Yes and no.. CAN is specified to keep working with one signal wire shorted to ground or 12V. If it where a pure differential bus, that wouldn't be possible.
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Hey, that would be interesting. If CAN does offer a somewhat "official" AC-coupled mode I might be able to dig into it. Have to google for it...
If the controllers don't support it, oh well. Most of the time we do such buses sans controller anyhow. But it would be nice not to stray too far from the officially blessed modes of operation.
for NXP's CAN controller that supports the mode. Philips previous CAN controller also supported it IIRC.
I've never heard of the mode being used but the fact that Philips retained the mode in the new chip may indicate someone uses it, or maybe it was just cheaper to keep it.
Robert
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To be precise: that's the specification for the low-speed, a.k.a. fault-tolerant CAN PHY layer. Which only works up to a certain speed (250 kBits/s if memory serves).
High-speed CAN is pure differential, low-speed isn't.
We don't need it. But we always try to stick closely to some kind of established protocol. A nice feature of an established protocol is that many uC contain a multi-purpose comms block that supports 2, 3 or sometimes even 4 methods.
One of the salient features of CAN is they do collision detection bit-by-bit. Basically, during the packet header a transmitter backs off if it sees a dominant bit (a 0) being transmitted when it is transmitting a recessive bit. This is cool because the transmitter with the highest priority message automatically wins, and gets to transmit immediately. Compare this with the inherently peer-peer nature of Ethernet, which detects a collision at the packet level then backs off for a random interval.
This collision detection puts a number of restrictions on the bus as far as propagation delays and noise (and a requirement that any physical layer be wired-or in one way or another), but the no-delay priority arbitration makes it easy to build a soft real-time bus.
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