2 wire rs485

or

I stand corrected. Funny when you look at it: CAN is just a high speed version of the old fashioned current loop using an opto-coupler...

Meindert

there are at least two significant differences:

The current loop is usually not terminated with the line impedance. Therefore you can't make it very fast.

And the current loop is usually no bus with several devices since the receiver takes all the _energy_ of the signal.

Oliver

What does that have to do with anything?

Does dominant/recessive map into 0 and 1? What is the noise margin of CAN?

You are mistaken. I was responding to someone who said that it was necessary to provide some form of bus arbitration (such as collision detection) to use a multi-party line such as RS-485. In reality, that is not needed since the sharing methode can be built into the protocol for the typical, simple RS-485 application. I was pointing out that there *are* devices which include a CD function. I only found one, but one is more than zero.

I still ask, what does that mean? Are you suggesting that if the short is transient it could be missed?

Rick

In current loop systems with more than two stations, you simply put them in series. All inactive stations are in the idle (mark) state conducting current through the loop and only the active transmitter disrupts the loop current.

Serial connection of current loop stations is more than 150 years old in telegraph stations along the early railroad lines. It is from those days that we have the expression mark and space, i.e. the pen made a mark on a moving paper, when there was a current in a loop.

Paul

Read what Oliver wrote and read what you wrote and tell me what is wrong with this picture...

How do you connected multiple receivers in series while maintaining a match between the impedance of the line and the receiver impedance? In a voltage based system you can use a single load and match the impedance easily. Using multiple receivers make it hard to get reasonable voltage waveforms on each receiver. Putting multiple receivers in series in a current loop will mess up an impedance match in an analogous way. So how would you make this work in a high speed impedance matched system?

Virtually any impedance matched, differential system is about the same as what has been described for CAN, a current loop. This is because the propagation of the voltage wavefront requires a constant current as long as the wave is propagating, then the load impedance requires the same current to maintain that voltage on the load. That is what prevents the reflections.

If I understand Oliver correctly that dominant/recessive map to a 1 and a 0 state, then if CAN uses a 5 volt driver, it will by definition have half the noise margin of RS-485 because it is uni-polar rather than bi-polar. RS-485 has close to 10 volts of driver range to work with, +5 to -5 volts differential. If CAN is on/off, then it can only have a 5 volt range = +5 to 0 volts (assuming it uses the same 5 volt power supply). Is CAN a 5 volt interface? Does dominant/recessive map to the 1/0 state?

Rick

Paul: You certainly noticed the word "usually" in my post. And do you know of real world current loop "bus" implementations used today where the idle station also receives a signal? How many bidirectional stations can participate?

With integrated circuits, it's harder to make a low impedance current receiver (with good common mode suppression) than a high impedance voltage receiver.

Well, with current sensing relays, I might consider a current signaling bus. A bit hard to operate it even at the lowest usual CAN speed...

Nothing. Basically both methods are possible.

theoretically you can make also very low impedance current receivers.

It's just harder to do.

That's true as long as the RS485 line is driven actively.

As I wrote, the problem with RS485 is the idle bus. There was a migration from a two state system to a three state system, and the designers didn't adapt the the thresholds.

Please note that this was not necessarily a fault of the designers of RS485, because you can have arbitration methods where the undefined state of the undriven bus doesn't hurt. For example "DIN Messbus" used a RS422 signal from a master to slaves for arbitration, and the slaves responded via RS485 (no, I never used DIN Messbus).

But some people wanted to use RS485 as single wire (pair) bus and needed to invent kludges (bias termination etc.) to make it work.

BTW: CAN is also not a perfect solution for every problem. It's limit is very high speed over long distances due to it's non-destructive arbitration method.

Oliver

When I wrote that RS485 noise margin of the idle bus is much worse than CAN, you replied that TP differential signaling mitigates noise - but that's irrelevant since CAN does the same.

I explained this in the post you have replied to in lines 22 to 24: "The cable is driven actively to the dominant state. The recessive state is undriven, 0V."

You can look also here:

As I'm at home today, I have no data sheets present. You will be able to check this yourself?

Maybe. You have to wait the time the signal needs to travel to the other transmitter and back again. Read my other post: "100m forth and back with 0,5c results in more than one microsecond delay".

During this interval, you simply can't detect a collision. With no system (well, _maybe_ quantum physic might help using photon pairs but I'm not sure about).

And as I wrote, there might be conditions where you can't even detect a static collision state due to driver strength matching and line impedances.

Oliver

... snip ...

Makes sense. Now, after using them for 50 years, I will finally know!! This may well help me remember which marks a 1 bit.

It is just one or two years ago when I told youngsters how to connect seven Premid (RF-tag) readers with current loop interface into a single curent loop, in which the master polled the readers. In fact, it worked better than I had expected, since apparently the transmitters used a FET instead of a bipolar, since the voltage drop was very low across the transmitters. The number of stations depend on the loop voltage and the breakdown voltage of the (active) transmitter.

I have never said that the 0-20 (or 0-60) mA current loop would be suitable for high data rates, but certainly sufficient for those readers (9600 bit/s) or for the telegraph key and pen system used in early telegraph systems. The problems with the mismatched current loop systems became apparent in the first transatlantic telegraphic cable.

Paul

interesting. The stations send _and_ listen to the bus traffic alternatively? I guess they are bus powered and therefore have no need for isolation, since they have no other connection.

Oliver