Re: 2 wire rs485

nemo wrote:


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

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.

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


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

Nothing. Basically both methods are possible.

How do you connected multiple receivers in series while maintaining a
match between the impedance of the line and the receiver impedance?

theoretically you can make also very low impedance current receivers.

It's just harder to do.

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

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 Betz, Muenchen (

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