Re: RS485 chip LTC491; optocoupler MOC5007

On 18/02/2011 08:24, Oliver Betz wrote:
Steve at fivetrees wrote:

[...]

>"Correct termination" is a can of worms. Almost all current RS485
>devices have the A/B circuits reversed re the spec. Also, simply
>terminating across the A/B lines will result in an indeterminate state.
That's why I consider EIA-485 broken by design.

Nothing wrong with the spec. Plenty wrong with the devices which followed.

Although - I presume your point is that, with termination and no bleeds, there's that indeterminate state. Yes, the specs could have been clearer about this. But it's not *that* hard. I've been using RS485 without problems (apart from those danged chips) for nearly 30 years now... (Eeeeep.)


>You need some bleed current in there to ensure a MARK state when idle.
>Hint: AC-coupling the termination often helps with the bleed current.
Do you have a working real world example?

I can't imagine that AC termination works if the bit rate is not way
beyond the propagation time. Either it's time constant is too short
for long cables, or it's too long for very biasing patterns.


I fear you misunderstand me. Given the lowish value of the terminator, the bleed to/from rails results in fairly high DC current. A transmission line is only an issue with AC ;). So, AC-coupling the terminator resolves the termination issue, while resolving the DC current issue.

There was an app note from my old company detailing this online, but they're rebuilding their site, and it's now "under construction"... gah...

Steve

--
http://www.fivetrees.com
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Relevant Pages

  • Re: RS485 chip LTC491; optocoupler MOC5007
    ... Almost all current RS485 devices have the A/B circuits reversed re the spec. ... AC-coupling the termination often helps with the bleed current. ... From experience, you can probably get away with 4800baud over that distance, if you're lucky. ... >> A fellow hardware engineer tells me that the RS485 chip and optocoupler may vary significantly from chip to chip and also over time/ temperature so that predicting a reliable baud rate is difficult. ...
    (comp.arch.embedded)
  • Re: RS485 chip LTC491; optocoupler MOC5007
    ... devices have the A/B circuits reversed re the spec. ... AC-coupling the termination often helps with the bleed current. ... optocoupler may vary significantly from chip to chip and also over time/ ... temperature so that predicting a reliable baud rate is difficult.<< ...
    (comp.arch.embedded)
  • Re: RS485 chip LTC491; optocoupler MOC5007
    ... "Correct termination" is a can of worms. ... devices have the A/B circuits reversed re the spec. ... optocoupler may vary significantly from chip to chip and also over time/ ... The tests I did were with termination - a 150 ohm resistor ...
    (comp.arch.embedded)
  • Re: RS485 chip LTC491; optocoupler MOC5007
    ... Nothing wrong with the spec. ... AC-coupling the termination often helps with the bleed current. ... beyond the propagation time. ... terminator resolves the termination issue, ...
    (comp.arch.embedded)