Re: RS485 chip LTC491; optocoupler MOC5007

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.

I disagree. Requesting a DC bias was the wrong way IMNSHO. The better
design choice is to make 0V a defined state, as CAN does.

Then I can use passive terminations at the ends of the bus and I don't
need to select a bus node to provide DC bias.


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.

That's what I understood. I think it doesn't work under the conditions
I pointed out.

Oliver Betz, Muenchen (