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Topic: RS485 why? (Read 1 time) previous topic - next topic

MarkT

50 transceivers all with 120 ohms?  That's 2.4 ohms across the bus lines.  How can that work?
Are they actively terminated perhaps?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Watcher

#16
Dec 07, 2017, 09:14 pm Last Edit: Dec 07, 2017, 09:18 pm by Watcher
50 transceivers all with 120 ohms?  That's 2.4 ohms across the bus lines.  How can that work?
Are they actively terminated perhaps?
You shouldnt forget the characteristic  impedance of the transmission line. It is not as if they were connected all directly in parallel. The point here is to achieve impedance matching and hence max transfer of power. (https://en.m.wikipedia.org/wiki/Impedance_matching)

Activily terminated? You mean the termination is not always connected?


ron_sutherland

#17
Dec 08, 2017, 07:08 am Last Edit: Dec 08, 2017, 07:25 am by ron_sutherland
The signal should be above +/-400mV if I recall, and bellow +/-200mV it is undefined. The special fail safe transceivers... wait is that why it works? Yep, the MAX3085 is one of the failsafe parts, it sees everything above -50mV as a HIGH. Are there resistors between A and +5V as well as between B and 0V. I guess that would do it.

What I am doing is just as outlandish, I turn everything off to send a HIGH so on the pair, 0V is a HIGH bit and -3V is a LOW bit. The transceivers seem to be cool with the HIGH (so to speak) and so is the battery.

C4m4l340

Thank you all for your opinions.
I was able to put them to work on breadboard.

Now i am implementing the rest of the program.
After new year, i believe its all ready to test with a 50m cable :)

Thank you all

Watcher

The signal should be above +/-400mV if I recall, and bellow +/-200mV it is undefined. The special fail safe transceivers... wait is that why it works? Yep, the MAX3085 is one of the failsafe parts, it sees everything above -50mV as a HIGH. Are there resistors between A and +5V as well as between B and 0V. I guess that would do it.

What I am doing is just as outlandish, I turn everything off to send a HIGH so on the pair, 0V is a HIGH bit and -3V is a LOW bit. The transceivers seem to be cool with the HIGH (so to speak) and so is the battery.
This is very interesting. ...i might give it a try too!

justPassingThru

#20
Today at 12:00 pm Last Edit: Today at 12:36 pm by justPassingThru
I was able to put them to work on breadboard.

You said you were using three, so did you remove the 120Ω terminating resistor from the middle one? (That would be R7 according to this pic.)

Aside: With three, if you look at the diagram here labelled "Typical Application", seems to me that any two are the end and the other one is the middle one, just depends how you stretch the wires out. If you swing that middle one from the bottom to the right, and swing the right-hand one round to the top middle, the circuit's the same but the "straight" bit  now joins the left-hand one to what was the middle one, and the old right-hand one is now the middle. So with three should it work with the resistor scraped off any one? Ignore that: I looked again and saw the twisted pair is the long run from left to right, and the middle one is on a short tap.

Aside 2: Seems bl00dy stupid to me that the unit has the resistor hardwired in so all of them are terminators unless you break it. Putting an smd resistor back is beyond my ability. A jumper would have made life a lot easier, even if it upped the price a wee bit.


C4m4l340

Aside 2: Seems bl00dy stupid to me that the unit has the resistor hardwired in so all of them are terminators unless you break it. Putting an smd resistor back is beyond my ability. A jumper would have made life a lot easier, even if it upped the price a wee bit.


This are prototype modules, is normal to have those resistors.
If moving to a production environment:
1) Use your own RS485 implementation and diagram
2) Adapt the modules

My option, i think will be, remove the resitor on both slaves, and set a jumper, because i will need to "create" a board for slaves.

But we need always to keep in mind the modules sold everywhere, are mainly to build prove of concepts

justPassingThru

This are prototype modules, is normal to have those resistors.
If moving to a production environment:
1) Use your own RS485 implementation and diagram
2) Adapt the modules


Yeah that makes sense.

Did you remove the resistor off your middle one though?


C4m4l340

Not yet, but i will when doing the final tests.

The final slave boards will have 2 RJ45 females on PCB.
between them i will put the resistor and a jumper.
On the end board (nr2), the jumper will be CLOSED to turn de resitor "on".
On the middle board (nr1) the jumper will be OPEN to turn the resistor "off".

That is the ideia. In some time, we will find if it works... ehehhe
i am documenting the entire project on another post.


justPassingThru

Not yet, but i will when doing the final tests.
Righto. But meantime I assume it's working with the terminator in place on the middle board?


i am documenting the entire project on another post.
Yep I saw: I actually came here from there.


C4m4l340

#25
Today at 01:07 pm Last Edit: Today at 01:13 pm by C4m4l340
Righto. But meantime I assume it's working with the terminator in place on the middle board?
Correct.

At the time, i only have 2 arduinos for testing.
I had i 3(borrowed one). With 3, i was able to put the circuit running with the builtin resistors, and additional 120ohm terminator resistores.
I am using the bus lines from breadboard, and 3 arduinos all of them with resitors and 2 other resistors and the end of fisical bus lines.

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